I'm not sure I understand what you mean. Can you explain this?
Assume you want to spend $120k per year and you are taking the money out of your traditional IRA, which is taxable income. In order to have 120k left after taxes, you need to "gross up". To do that, you can do the following: (assume Married Filing Jointly)
1. In 2020, the 10% bracket applies to income up to $19,750. That is $1975 in taxes.
2. In 2020, the 12% bracket applies to income from $19,750 to $80,250. So that's another $7260 in taxes.
3. Now the slightly tricky part - you need to figure out how much to take in the 22% bracket. You're going to need at least $39,750 to get from $80,250 to your desired spending of $120,000. And you're going to need to take another $9235 to pay the taxes in the 10% and 15% brackets. Which means you'll be drawing at least $48,985 subject to 22% taxation. Already, your draw is up to $129,235
4. Now, you'll need to figure out the taxable amount to take in order to end up at $129,235. To do that, first take the standard deduction of $24,800, giving you $104,435. Subtract out the start of the 22% bracket ($80,250) to give you $24,185. Gross that up for taxes using the following equation: gross amount = net amount/(1- tax rate) = 24185/(1-0.22)=$31,006
5. Now, add the standard deduction back in, so $31,006 + $24,800 = $55,806. Add the beginning of the 22% bracket ($80,250) to get
$136,056. That is the spending you need to put into FIRECalc.
6. As a check, let's do your taxes. AGI = 136,056. Taxable income = 136,056 - 24,800 = 111,256. Tax in 22% bracket = (111,256 - 80.250) X 0.22 = $6821. Plus tax in 12% bracket ($7260). Plus tax in 10% bracket ($1975). So total tax equals 6821+7260+1975 = 16,056. So if you take $136,056 out of your taxable IRA, you'll have $120,000 left to spend after taxes. Your effective tax rate is 11.9%
7. You'll also need to figure your state income taxes (if any) and add that amount to your spending as well.