Utilizing iteration in conjunction with Excel function

I need to write some sort of Excel construct (a function?) that will return the federal and state taxes that will be incurred when I withdraw a specified level of spending money from my IRA. This sounds easy, except that the actual taxes incurred are dependent upon the total amount of money that is withdrawn from the IRA--and this total amount includes both the spending money and the money needed to pay the taxes. So computing the taxes cannot be done as a single calculation, but must use some sort of iterative approach. Ideally I would like to start with a spending value, (SV), that would be the amount of money that I want to spend in a year, not counting taxes. The function (or functions) would then go through some sort of iterative calculation process that would compute the total spending level including taxes, (TS), the federal tax, (FT), and the state tax, (ST). These values would have the relationship that TS=SV+FT+ST. When the iteration was finished, the federal tax and state tax amounts would be consistent with the taxes that would be due for this level of total spending as determined by the normal tax tables for both federal and state taxes. I am not concerned that the tax figures be absolutely exact, so a lot of iteration is probably not necessary. Any recommendations for how I might best do this?