The underpayment rate was 7% for most of 2023. Now up to 8%. The penalty is calculated by looking at the balance due and the time period for which it was owed. So your 1st qtr payment due April 15th for 25% of your liability will have interest calculated on just the 25% owed from the April 15th due date until the date you finally pay it. Similar deal for the other estimated tax deadlines. The total penalty won't equal the 8% as you didn't have the full liability due for the entire year.