Is that why our number don't agree? Read back through your post. You never mention that you were changing A's income from $25,000 to $50,000. Making A & B's income unequal makes it much messier to write the post below but I don't see how uneven incomes change the point. The approach you are describing makes both A & B working look like a better deal financially than it actually is.
In your example the couple has $50k $75k of total income, an exemption of $20k and pays 30% tax on the remainder, which produces a take home pay of $41,000 $58,500.
If the tax exempt space were divided evenly proportionally to income (which used to mean the same thing the same until you changed A's income) between the two of them, each A would have an exemption of $10k, pay 30% of $15k $40k in tax ($12k 4,500), and have a take home of $20,500 $38,000. This suggests that if A either spouse wanted to quit working for any reason, they would need to either decrease their living expenses by $20,500 $38,000 or save up enough money to replace $20,500 $38,000 income.
However if A either spouse actually quit working, the tax owed by the couple would be only 30% of $5k = $1,500. This means the couple would have a take home pay of $23,500 and only needs to either decrease their living expenses by $35,000 $17,500 or save up enough money to replace $35,000 $17,500 of income before A or B could stop working.
Copy and paste here to run the same analysis for B. Like for A, B is $3,000 better off not working than would be suggested if the tax exempt space were divided equally (or proportionally to income if income is unequal).
By dividing the tax equally proportionately A & B end up thinking they need to work longer before either one of them can FIRE than they needed to.
Anyway, at this point we just seem to be repeating ourselves over and over, which isn't going to get us anywhere and is probably getting tedious for the other people on this thread. I suspect we just perceive the world in fundamentally different ways. Good luck to you.
