@Opus 17 your explanation is correct. I run into this myself.
The EASY way to avoid all the hassle is to pay the Conversion $$$$ times your tax bracket by Sept. 15. Just pay it as an estimate at the IRS (and State if necessary) website.
Alternatively, filling out Form 2210 and checking Box C indicates "lumpy income" when completing your 2023 tax return. It will minimize any penalty (and might eliminate it).
Form 2210 and 2210ai is a pain because in effect you have to determine your income for each of the 4 periods of the year (YTD March, YTD May, YTD August and then of course the total year). It's a lot of math and a big spreadsheet to determine when which income is received when. If you get a lot of dividends for varying pay dates - what a pain!!!
if you are not concerned with the cash flow and lost interest between Sept 15 and April 15, just pay it in Sept. Another approach is do the conversion on Sept 1 as the estimated payment is not due until Jan 15. (note above the Sept 15 estimate is predicated on YTD August income 😀)
