Hog, income tax is so complex and different from user to user that FC really can't factor that in as a general tool. Not to mention that fact that IRA withdrawals are subject to income tax as ordinary income, while personal after-tax savings is not. No way FC can know this.
Basically, when you put in how much you need from your nest egg, use your gross (pre-tax) amount for your expenses. You'd estimate that based on your income level, deduction status, etc. as you normally would. Plus, you don't need to "gross up" your personal after-tax savings withdrawals.
So if you need to spend $50k a year in retirement, and you need to "earn/withdraw" $60k to leave $50k after taxes, your income need is $60k for FC purposes.
San Francisco Area
ESR'd March 2010. FIRE'd January 2011.
As if you didn't know..If the above message contains medical content, it's NOT intended as advice, and may not be accurate, applicable or sufficient. Don't rely on it for any purpose. Consult your own doctor for all medical advice.