First, I just rechecked my AGI for 2023 and it's below the $150,000 trigger that sets the Safe Harbor trigger at 110%. So I just need to pay 100% of last years taxes this year to be safe.
I use the IRS online payment site for all my estimated payments. Let's assume my 2023 tax that I've paid in estimated payments all last year was $16,000. So I'll log on and tell them to ACH my 2024 April estimated payment on the 15th and to take $16,000. Since 2023's tax was $16,000 that will be 100% and after April I will have paid my Safe Harbor amount. Then, since my total 2024 tax (which I'll pay in 2025) is, let's say $40,000 I will have to pay the other $24,000 by Y/E 2024 (Jan 2025). I will make three estimated payments of $10,000 (6/15), $10,000 (9/15) and $5000 (1/15/25). Things will change in 2025.
In the past I paid estimated taxes at around 40%, on the first and 20% each after. Sometimes when they were low I just paid the total by the September payment. But now that my RMD's are growing as I age I'll go back to 4 payments. And, starting in 2025 I'll probably look to just having my pension, Social Security and RMD payers take out withholding to simplify life, though I'll probably set the actual amounts myself so I don't come up short.
