@taxdean - let's use a more concrete, simple example. Let's say you do a Roth conversion in April, 2022 for a $300,000 and that is the only income you have for the year. Let's not worry about any safe harbor based on the prior year's tax liability.
if you wait until January 15, 2023 to make an estimated tax payment - and let's say it's EXACTLY the correct amount to pay the taxes that will be calculated on the tax return. there is going to be a penalty because the IRS would have expected you to pay those taxes quarterly over the course of the year.
if you make the payment on June 15, 2022- and let's say it's EXACTLY the correct amount to pay the taxes that will be calculated on the tax return in April of 2023. Again, the IRS would have expected you to pay it quarterly over the course of the year. But in this case, you get credit for paying it early in the year, so the penalty is less than in the paragraph above.
if you make the payment on June 15, 2022- and let's say it's EXACTLY the correct amount to pay the taxes that will be calculated on the tax return in April of 2023. You fill out form 2210ai and show that the income was all recognized in 2Q and that is the same quarter that you made the estimated payment. There would be no penalty. In this case, you get credit for paying it in the same quarter as the income occured.
