The line on the worksheet used to calculate the tax (line 7 or 21 depending on which worksheet is used) tells you what tax bracket your ordinary income is falling in but does not necessarily tell you your marginal tax rate due to side effects that can change your AGI by more than the amount of the incremental income, particularly Social Security income as NCperson mentioned. The way Social Security income is taxed, for a certain range of income you'll find that your marginal tax rate is 1.85 times your tax-bracket rate due to the increase in taxable Social Security income. Once you get to the point where a full 85% of your Social Security income is taxed your marginal tax rate will drop back down. There is really nothing new about this, it's been this way since Social Security income became taxable in 1984. You need to map out your marginal tax rate by testing incremental amounts of income and find the sweet spot for the amount of your IRA distributions, and you might find that it is beneficial to go higher to a range where your marginal tax rate drops back down. Of course you'll want to convert to Roth any of those additional amounts distributed from your traditional IRA that you don't need to spend on living expenses.
