I think paying 90% of last year's taxes is 'safe harbor' with IRS; paying that much this year means no penalties apply if you owe more at tax time. (110% if you make a lot?) So I divided my 2023 taxes by 4. Then paid 1/4 of 2023 tax amount by each due date, subtracting what went to them this year from my paycheck. By 4/15, 6/16, and 9/16 I sent an ACH payment directly to the IRS for the difference. Did the same for state taxes. It might get tricky when income drops after retirement, but for now hopefully it works!
