This is a very complex situation and while we do have some tools available to assist, we do not have a dedicated IRMAA calculator. We do offer a custom scenario input on our Free Tax Reform Calculator.
If you purchase the desktop version of TurboTax, it does include a "What-If" worksheet tool. For a Roth conversion, you could incrementally increase the amount of your converted IRA and observe how it impacts your tax bracket and total tax owed. Many financial services companies offer planning calculators, so you might check with the company who administers your IRA.
Hope this helps!
Cindy
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