401k to Roth Conversion - Tax Calculation?

[BrianF1]

I am using 2023 TT desktop and trying to do scenario planning for 2024.  I realize that isn't perfect, but should be close enough for what I'm doing.  I am planning a 401k conversion to a Roth IRA in late 2024 and trying to understand the tax ramifications.  I understand that a 401k conversion to Roth is taxed at my marginal tax rate, but am having difficulty understanding the actual calculation steps.

My income is in Dividends (mostly qualified) and Capital Gains.  I first do a scenario with no conversion, and then one with a $30k conversion, so that I can compare the impact.  To examine the tax calculation, I am going to the Qual Dividends and Capital Gains Tax Worksheet in TT.  

All looks fine in the first scenario, without the conversion.  When I add the conversion on a 1099R, TT appropriately adds the $30k to Taxable Income as I expect.  On the Qual Div and Capital Gains tax worksheet, TT then subtracts $89250 (MFJ tax bracket for Cap Gains) from Taxable Income and multiplies by 15% (lines 17-18).  Thus, the $30K conversion is being implicitly taxed at 15% since it isn't excluded from Taxable Income in this step.  TT then does another tax table calculation for any non-Qual dividends + $30k conversion less Itemized Deductions.  The tax table rate is about 11% blended.  Thus, the conversion is taxed twice, once at 15% and then again via tax table.  The outcome is my conversion is taxed and the marginal rate when I compare the 2 scenarios is about 26% which is not what my marginal rate should be.

Can you help me with the calculation logic?

[AmeliesUncle · Accepted solution]

I think you are misreading the worksheet.  The worksheet is ONLY taxing the capital gains and dividends.

However, you are correct that it could effectively be a very high tax rate.  While the Roth conversion would be taxed at your regular tax rate, that 'extra' income could cause your capital gains/dividends to go from the 0% tax bracket up to the 15% tax bracket.  That means the direct tax on the Roth conversion PLUS pushing your other income from 0% to 15% could effectively result in a very high tax outcome.