How do I do projected taxable income and taxes for 2021for purposes of paying estimated tax in Dec 2021 through RMD withholdings?

[rjs]

There's really no good way to do what you want in TurboTax Online. If you use the CD/Download TurboTax software you have two choices.

If you have the 2020 CD/Download TurboTax software you can use the What-If Worksheet in forms mode to estimate your 2021 tax. At the top of each column in the worksheet there is a box to check to use 2021 tax rates.

The other option is to wait until the beginning of December. By then the preliminary 2021 CD/Download software will be available. Install the software and use it to prepare a complete 2021 tax return, using your best estimates of your final income for the year.

Either way you still have time to make your IRA withdrawals in December and specify the amount of withholding that you need.

[Critter-3]

Use the Tax Withholding Estimator on the IRS web site 

Tax Withholding Estimator