Here's some info on Form W-4P from the IRS and also a video on Understanding the New IRS Form W-4P.
TurboTax does currently have a W-4 Calculator.
If you're a retiree receiving income from Social Security, pensions, etc., the first year you file with your all your retirement income, see what tax bracket you're in. If you're in the 15% tax bracket, for example, if you withhold 15% from your largest sources of income, you should be good. Remember you still get the Standard Deduction (at least) off the top of your income.
@Dave106
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