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.