The two basic laws in circuit analysis, Kirchhoffs Voltage and Current Laws,

[KVL] Voltage across the same pair of points is the same no matter what paths you take

[KCL] Current stay the same along the same path

are often taught in basic circuit analysis, but most of the time, they taught it in the context of nodal analysis, which you have a little more complicated meshes with multiple theoretical power source (voltage or current) that simple series/parallel circuit rules are not enough to solve the puzzle.

However, these two fundamental concepts are useful to develop insights that help you estimate quantities in a circuit quickly like a pro.

Kirchhoffs Voltage Law [KVL] can be applied to a parallel circuit of 2 branches (often the case when measuring additional loading effect). Let say the two branches are applied (loaded) at a voltage output , which might change depending on the branches (loading).

You can exploit the algebra to quickly calculate the current of any branch without first computing the overall resistance or current:

Kirchhoffs Current Law [KCL] is useful in analyzing energy loss over resistance in wires . For example, in high school physics, we discuss why we have high voltage power lines for bulk energy transmission despite it’s more dangerous. The traditional explanation is

so the lower the current is (which can be done through stepping up the voltage, traditionally done with AC signal through transformers, to maintain the same power). But how about other form

Technically, it’s possible, but you have to be very careful that the voltage we are talking about is across the wire with resistive losses , NOT the load voltage .

changes depending on the output load , so you have to derive the assuming an arbitrary , which will happen to cancel itself out and end up the same as if you think of everything in terms of current first:

So the bottom line is that most of the time, it is easier to think in terms of current in most circuit analysis because current won’t change along the same path. This is especially true when your problem has varying impedances/load which will disturb the voltage.

Of course, if the problem screams direct application using KVL, don’t go all the way converting it back to current. You will find the current-first approach useful when we get to semiconductors like diode, voltage references, BJTs,.

I usually think of voltage as a consequence or effect of current flushing into a transducer (e.g. resistor), so it’s subjected to change and therefore messy to use when solving circuit puzzles. Solving circuit analysis problems are often an exercise of identifying invariants and inferring the remaining quantities.

INTRODUCTION TO HOLOLISM
A NEW PHILOSOPHY FOR A NEW CENTURY
ORIGINAL BY
THORIUM HOLMIUM (ThHo) SULPHATE
(B. SC, HKU; CERT ED., HKU)
(19XX-20XX)
EDITED BY: SMALL POTATOES (B. PH. SC, HORNS., HK(CEE)= 9U)

Most often when we sell on eBay, we receive Paypal payments, so we ended up paying eBay’s commissions (also called the final value fee, FVF for short) and Paypal processing fees as a percentage of the sale.

Now eBay pushes out Managed Payments (MP) which combines payment processing fees with the eBay commissions (FVF) because eBay now manages the payment gateway. The rest is every time you sell something, you get a payout (sales – fees) deposited directly to your bank (it used to be collecting paypal balances then withdraw it).

They have a different calculation formula which they claimed the sellers are better off in most cases, but should we take eBay’s word for it? Regardless of whether you are enrolled in managed payments, the fee percentage for each sale depends on:

eBay store subscription level

category

It’s impractical to do the side by side fee structure comparison to see when we are better off for each sale, plus we cannot easily switch between the two plans.

It’d be very helpful if we can put the managed payment fee structure on the same form as the conventional eBay+Paypal fee structure, and figure out under what conditions we are better off or worse off with managed payments.

Initially I was ready to do the derivations to put both plans on the same scale, but I spotted that managed payment (combined) percentage is simply the vanilla (non-managed) eBay category final value fee + 2.35% payment processing fees! That’s how they’ve calculated the combined managed payments percentage!

This makes life a lot easier. Since I can factor out the 2.35% that applies to the whole sum (which also include shipping and sales tax) regardless of the fee cap, this works exactly the same as Paypal (which charges 2.9%) and we are getting a 0.55% discount in payment processing fees.

For managed payments, since we’ve already separated out the payment processing fees, the fee cap applies to the vanilla final value fee portion which is equivalent to the old eBay final value fee. Keep that in mind.

The part that has changed is the fee cap. The old way caps thecommissions/FVF directly regardless of product category, yet the new way (managed payments) caps the sale price that are charged commissions. This means the fee cap goes up or down a little depending on the final value fee percentage class applying to your sale.

eBay set the (commissioned) sale price cap to closely match the realized commission cap in the old way, for example non-store subscriber who will have their raw final value fees capped at $750 will see the same cap for the most common 10% categories (12.35%-2.35% = 10%) because the (commissioned) sale price cap is $7500, which $7500*10%=$750.

Big industrial equipment also have the same cap regardless because $15,000*2% = $300.

For non-store subscribers, 10% is the anchor (iso-fee-cap) category. So you are a little worse of with books (price cap +2%) and better off with musical instruments (price cap -6.5%).

For store subscribers, you get a bit more break over the fee-cap (lower) because your final value fee percentage is lower than the anchor (likely they chose a breakeven point at 10% when determining the sales price to cap final value fee over. Just easy numbers, not rocket science):

Managed

Conventional

Discount*

Common (9.15%)

$2,500*9.15% = $228.75

$350 (Small stores)
$250 (Big stores)

$121.25 (Small stores)
$21.25 (Big stores)

Heavy gears (1.5%)

$15,000*1.5% = $225

$250

$25

Books (12%)

$2,500*12% = $300

$350 (Small stores)
$250 (Big stores)

$50 (Small stores) -$50 (Big stores worse off)

Guitars (3.5%)

$2,500*3.5% = $87.5

$350 (Small stores)
$250 (Big stores)

$262.5 (Small stores)
$162.5 (Big stores)

I call Basic/Premium ‘small-stores’ and Anchor/Enterprise ‘big-stores’.

So here are the observations, which is all you need to know:

Under managed payments, small-stores gets a better deal than big-stores, because the advantage of the $100 lower fee cap with big stores are eliminated with variable fee caps.

The breakeven line for small stores ($350 cap) is 14% fees ($350/$2500), which the highest category is 12% so far. This means small-stores are ALWAYS better off switching to managed payments.

The breakeven line for big stores ($250 cap) is 10% fees ($250/$2500), which the only category above that right now is books (12%). Big-stores selling BOOKS are worse off with managed payments.

So in other words,

everybody is better off with managed payments (fee-wise) EXCEPT big-stores selling books

under managed payments, there’s no final value fee advantage for being a big-store

* Remember you got 0.55% discount over the payment processing portion of the fees too and is not shown here since we’re just talking about savings in vanilla final value fees.

As far as books (12%) is concerned, if you are a big-store owner, your raw commissions cap raised from $250 to $300 because $300 = $2500 * 12%. But if you factor the 0.55% discount, if the sale price is $x,

Since the raw commissions stops at $300 ($2500 * 12%), the additional $50 cap increase starts to get offset (and turn out positive as the processing fee discounts outweighs the commissions cap increase) when the payment processing discounts ABOVE $2500 covers all of it:

So the range of sale price x which the only setup (books for big-store sellers) can do worse is

There are some ambiguities (technically incorrect documentation) on eBay’s website which implied vanilla final value fees (portion) are charged for sales tax. I made a sale and checked the numbers and it’s not true. Only the 2.35% payment processing fee portion is charged against the sales tax (like paypal for handling extra money), the category final value fee (in my case 9.15%) is not applied to the sales tax.

They actually meant “the 2.35% payment processing fee portion” when they said “managed payment final value fees”. This is also part of the reason why I wrote this article, because they do not use the language that conceptually separate the two portion of the combined final value fees (vanilla final value fee + payment processing fee) in managed payments, thinking that they are simplifying the math for sellers, without realizing if the two concepts really fused into one, they’ll be shortchanging sellers over sales tax.

The $0.3 fixed per-transaction fee applies to both managed payments and the conventional way (Paypal also charge +$0.3 fixed per-transaction), so nothing has changed.

It’s very common to have a percentage fee (called taxrate below) slapped on our earnings:

Commissions (e.g. eBay final value fees)

Sales Tax

Incremental income taxable within the same bracket.

Many adults cannot directly figure out how much EXTRA do they need to earn to get the target amount of that are rightfully theirs (after fees).

They are missing out these handy intuitions:

If your EXTRA earnings are taxed at 40%, not wasting an extra $100 is almost as good as earning $166.67!

You always undercharge the fee overhead if you just multiply your target amount with it (intuitive but WRONG). The amount you get shortchanged goes up like a rocket for rates past 10%!

All you need is grade school math to figure it out, but the results are highly non-linear and are not easy to remember. This means the tutorial below makes an excellent discussion material to motivate mathematics education in grade school!

If you want to copy or use the materials, just remember to reference “Rambling Nerd with a Plan” blog page or “Humgar LLC”. You don’t have to ask and you are welcome. I certainly appreciate comments about this tutorial.

Before I start, hints for working with percentages:

divide percentages by 100. Think multipliers/factors in your head.

a factor of represents the full amount

division by is multiplication by reciprocal

preferably work in terms of shrink/expand factors (multiplications only) over the FULL amount, rather than working out the excesses (additions/subtractions)

Assigning intuitive names are very important for interpreting algebraic expressions with familiar life concepts!

Let’s define these variables first:

is the target price you want to get AFTER the fees

is the damage you should charge

is the rate (fee/tax percentages)

is the excess that goes to the fee/tax collector

These derived variables will be discussed in the body of the tutorial

is the shrinkage (multiplier), where

is the magnifier (multiplier),

is the compensation (multiplier) for billing,

If somebody pays you the total damage before tax/fees (at rate ), you are home free with after subtracting the fees from

In other words, you the FULL damages gets shrunkby a factor which becomes the target amount you get to keep for yourself.

Don’t be scared by the algebra! It’s just the other way of saying, if your tax rate is 30%, it means you get to keep 70% of your income, because 100%-30% = 70%, so the shrinkage is .

Since you have the target price in mind and want to undo the shrinkage (multiplication) to calculate the full amount you should charge , you divide the target price by the by the same factor to magnifyit back:

It is NEVER helpful to directly multiply the fee/tax percentage with the target price like what most people would intuitively do. We can think of the division above as multiplying by the reciprocal of the loss factor or , which we will call the magnifier:

An alternative view is to break the damage into targetprice along with excess to bill the customers for the fees/taxes (which goes to the collector):

My preferred way to calculate the excess is simply compute first by finding calculating the magnifier , or simply dividing the target price by the shrinkage

then subtract the target price from the damage to get the excess:

which you can write it in terms of target price which you know so you don’t have to work out the total damages first:

We can define compensation (multiplier) as how much you should multiply the target price with:

to recover your fee/tax costs :

e.g. if you need to charge the customer 1.43 times to breakeven after fees (given a 30% tax/fees rate), the overhead ‘item’ on the bill should be 0.43 (43%) of the target price. For a $100 item, you’ll need to bill your customer $43 more just to cover a 30% fees.

In summary, we have

with that can be figured out quickly as

where and are both bounded between 0 and 1 which adds up to 1:

Take an example of 30% tax rate (), the shrink is 70% (), because the factors are complements to each other which adds up to 1.

If you want to write it all in terms of shrinkage :

Since , we can rewrite the compensation (factor) in these complement terms , which I think it’s the easiest way to remember:

or simply the rate divided by shrinkage ,

You can also write it in terms of fees/tax rate in full:

e.g. if you have a 30% tax rate , you can find the compensation(factor) by calculating , because it’s simply 30% over 70%.

To estimate the overhead charge to offset the fee percentage, or compensation by multiplying with the target price :

WRONGway

CORRECT way

The gap(amount you get shortchanged by computing compensation wrong) is:

The numbers look close enough (for the intuitive but wrong way) when it’s like around 10%, but your loss (overheads not charged) shoots up (non-linearly) as the fee/tax percentage goes up beyond 10%!

Fee percentage

Correct overhead % to charge

Undercharged

5%

5.26%

0.26%

10%

11.11%

1.1%

20%

25%

5%

30%

42.86%

12.86%

40%

66.67%

26.67%

50%

100%

50%

Intuitively, you need to charge DOUBLE the target price if half of it gets taxed to breakeven, NOT adding half of the target price as the overhead! The FULL target price itself is the right overhead to charge for a 50% tax rate!

Since the numerator is bounded because , the denominator can blow up (the overhead needed to charge your customer) as it gets small (i.e. high tax/fee percentage). Here’s a plot of

.

e.g. A 66.6% imports tax rate means giving two cars to the government when you import one!

To push it to the extreme, 90% tax rate would mean 900% overhead, which means if you buy one car, you give the government 9 cars, assuming they don’t tax themselves. Your estimates are off by 10 times if you use the wrong method to compute the excess making up for the tax that gets charged.

If people can think in terms of the excess you will need to earn EXTRA to offset the tax rate, you will think twice about letting the government dip in another 2% or so. That’s why welfare-state through taxes can be highly counter-productive when it get past certain point.

Advanced analysis for the intrigued

Just for fun, let’s investigate the impractical case (communist tyrants) when tax rates gets to 100% and above ():

Fee percentage

Correct overhead % to charge

Undercharged

90%

900%

810%

100%

200%

-200%

-400%

300%

-150%

-450%

-100%

–

When charged 100% tax rate, the government seize any gains from your production. There’s never enough overhead you can charge your customer to breakeven.

The more perverted case is what happens when you go above 100%, say at 200% tax rate? When you sell $100 worth of stuff, you pay the government $200 to do so. You are paying a $100 penalty to work/produce $100 for the government. Twisted!

Then how do we explain the negative numbers in the compensation (factor) ? In reality, it means you ALWAYS make a loss no matter what. There’s no way you can bill your customer to make up for the loss.

In the unrealistic (unicorn) case, if the benevolent crazy dictator is willing and able to compensates your losses (at the sales tax rate), aka giving you money back every time you lose money,

means the the magnifier for sure when . It’s illustrated in the graph as a deadzone

:

which you can verify the same by solving this inequality

and realize you get when , which boils down to saying which is a contradiction.

That means to achieve target earnings under tax rates above 100%, you should charge the customer negative amounts (i.e. giving away goods + money) in order to get the negative taxes (subsidy).

e.g. At 300% tax rate (), it your sticker price (also target price) is $100, you’ll need to GIVE the customer $50 so the government will triple match your loss as a subsidy (which is $150), which is enough to cover the $50 giveaway (to the customer) plus the $100 so you’ll breakeven.

Having negative tax (subsidy) is a perverse incentive. Forget about maintaining a target earning (same as the original sticker price for the goods you are selling), just lose (give away) money as fast as you can to suck up as much subsidy as you can before the system collapse.

I still fondly recall my economics 301 (intermediate microeconomics professors) James Montgomery saying that “if you can graph it, it can possibly happen” in the lecture.

Even more sophisticated math for the geeks

Remember from above, the magnificationfactor can be rewritten as the reciprocal of shrinkage factor ?

which reminds me of geometric series

given ? For non-perverted cases, we are considering , which is well within the domain.

This gives an idea of approximating the compensation factor :

which after taking off both sides:

This explains exactly why people are wrong to just assume is just , because their false intuition kept ONLY the first order term in the power series approximation!

The gap (how much they are off with the WRONG way), is

So while dies fast enough (0.01) by 2nd order term, , … are still very significant up to say 6th order term which makes it off by a factor of .

Yes, you are off by 1% when the tax rate is 10%, but you can be wrong by a factor of times just multiplying the tax rate with the targetprice to estimate the amount of excess to collect to make up for the fees!

Summary

The total damage (bill) you should charge your customer can be calculated by dividing the targetamount (of money you want to home free with) by the shrinkage, which is (1 – tax rate). e.g. 30% tax rate means 70% shrinkage. Divide the targetby 0.7 to get the final bill

A shortcut to calculate how much you should put in (by multiplying with the target price) for an charge item on the bill to make up for the fees is (tax rate)/(shrinkage) e.g. 30% tax rate means 70% shrinkage. Multiply the targetby 30/70 (or 3/7, which is 42%) to get the excess which covers the fee.

Do NOT attempt to use the rule of thumb to calculate the excess by just multiplying with the tax rate unless it’s 10% or less. The ‘rule of thumb’ assumed (and thus the higher powers) are insignificant, which is nowhere true for 0.1 and above based on this graph!