just to echo the comment in the thread above
"Also note (for future conversions) that the timing rules work in your favor if you do the conversion between January and March. If you convert between January 1 and March 31, you don't need do withholding, and instead you owe 1/4 the estimated amount on April 15, June 15, Sept 15 and Jan 15. That allows you to hold some of the money back and invest it elsewhere in the mean time. "
so if you know next year you will do a Roth conversion, if you work out quarterly estimated tax payments based on the smaller of prior year tax (100% or 110% if AGI > 150k or 75k if filing MFS), or 90% of current year tax; then you won't need to worry about 2210 AI method as long as you've paid quarterly ES evenly and on time. You can also make the Roth conversion in Q1 rather than Q4 if you know how much you plan to convert, to take advantage of an extra year of tax free earnings. When you file for 2025 you can work out ES options under Other Tax Situations / Form W4 and Estimated Taxes in time for the Q1 ES due in April.
As for 2210 AI method, the level of pain depends on your situation, you basically need to calculate your AGI for the first 3 quarters (uneven dates - 3/31, 5/31, 8/31 - and 12/31 is your full year return anyway) plus any withholding, qualified dividends and LTCG. This method is covered when you file under Other Tax Situations / Underpayment Penalties. And then you may have to deal with equivalent process under state tax also. I ended up doing this for 2024 due to a December Roth conversion and large Q4 ES payment, and it was a lot of work but did reduce the default penalty; to avoid that mess again in 2025 I just did quarterly ES based on 2024 tax, and did a Roth conversion back in January.
