Just to clarify on the AI method where I said previously "full amount of your ES due to the Roth conversion to be paid in Q3" etc this may not be the case, depending your 2024 situation. In the AI calculations, your ES requirements are still limited by your 2024 tax safe harbor (see Form 2210 AI Schedule lines 22-27) and the annualization of a large income spike in Q3 will likely exceed the 2024 safe harbor amount, so the most you need to pay in Q3 should be 75% of your 2024 safe harbor amount.
Re withholding, if you weren't planning to make ES payments without the Roth conversion, and your withholding meets your 2024 tax safe harbor amount, then this is the simplest scenario, you won't need to pay additional ES at all regardless of the Roth, and just pay the balance due in April 2026.
Hopefully you can take advantage of the 2024 safe harbor one way or another. If it doesn't apply and you need to pay ES based on 90% of 2025 tax, I am not sure the exact math of the minimum you need to pay in Q3 vs. Q4, other than the total ES paid by Q3 should be not be less than 75% of the total ES due for the year.
If you have TT desktop you can run a mock return for 2025 figures using 2024 software - granted the calcs will not take 2025 tax brackets/deductions and "big bill" changes into account - you can then input different estimated tax scenarios and check the penalty outcome in Forms mode, and run the AI method under Other Tax Situations / Underpayment Penalty.
