Sharing Expenses Proportionately

The soon to be ex and I have been going back and forth on various topics in the yet to be signed separation agreement.

One item we’re discussing is splitting medical bills for the children, such as braces.
With the proposed child support and alimony, I’m essentially going to be contributing every other pay check.

For the purposes of discussion, I’ve used a hypothetical model to try to rationalize the dynamics behind the math.
My contention is that when determining the proportional split, the adjusted income should include alimony and child support but I’m probably wrong and would like some clarification.

For example:
Jimmy and Amy are brother and sister – the earners in this situation.

Lets say Grampa starts giving Jimmy $4 a week to rake leaves.
Grandma decides that it isn’t fair that Amy doesn’t have any money to buy stuff too and Jimmy is told to give $1 a week to her.
He’s okay with that – not that it matters.

Now at some point Amy starts doing chores and is given $1 a week. Grandma and Grandpa don’t really know about this money nor do they really seem to care.

Amy is now bringing in $1 a week + what Jimmy gives her (totaling $2). Jimmy is bringing in $4 a week but has to give $1 to Amy ($3).

They decide to buy a Guiney Pig together and it cost $2 a week in expenses. Amy only technically makes a Dollar a week according to gross income as reported to the tax collector. In determining Guiney Pig care, Jimmy’s contributions to Amy are ignored per being “alimony” paid in the same instant action. So, Jimmy’s supplement to Amy’s income doesn’t count for some reason. So Jimmy has to now give Amy another $1 so they can both contribute to the care of the Guiney pig equally.

Jimmy starts with $4 gives Amy a $1 for support and $1 for the pet = he now has $2 a week going into his piggy bank.
Amy has a $1 for chores, $2 from Jimmy going to her piggy bank.

Now the pet gets sick. The bill is $4 from the vet.
It’s decided that everyone pays a proportionate amount.
So to determine the amount they pay:
Does the system say that Jimmy makes $4 Amy makes $1.

Or does Jimmy make $4 - $2 = $2 and Amy makes $1+$2 = $3. I think the latter because, well – that’s the logical reality of the situation.
For tax purposes, Jimmy is allowed to say he only makes $3 a week since he was told to give Amy $1 (in alimony). The care of the Guiney pig doesn’t count as a tax deduction so he has to declare that he makes $3. Amy only has to declare that she makes $2 – chores plus the $1 Jimmy was told to pay.

If we take some people’s literal interpretation, we’d use the former:
Jimmy’s Income (times some amount) + Amy’s income (times some amount) = the vet bill amount
$4x+1x = 4 (vet bill)
X = .8
4(.8)+1(.8) = 4
3.2 + .8 = 4
So, Jimmy would owe 3.20 and Amy would owe .80. In this method, he doesn’t even get to reduce his income by the $1 (pet support). He’s going to spend 2 weeks crawling out from under the debt while Amy is free and clear.

This system completely ignores the fact that Jimmy is giving Amy $2 a week to level their combined ability to pay bills.

Or is it done like this:
Adjusted income (times some amount) + adjusted income(some amount) = the vet bill
4 - 1- 1 (x) + 1+1+1(x) = 4
2x+3x = 4
X= .8
2(.8) + 3(.8) = 4
Jimmy pays: $1.6 (so he still has some money left to buy candy) (.40 cents)
Amy pays: $2.4 and she too has some money left to buy candy (.60 cents)

Any insight would be appreciated.
Thanks in advance.

How to parties agree to split expenses varies per family. You can agree to split everything evenly or based on income; it really just depends what works for both parties.