TT uses Form 2210 to calculate the penalty. If you are on desktop go to Forms mode to the penalty line on 1040, double-click it to the 1040 worksheet and double-click it again to bring up Form 2210 which will show the minimum safe harbor amount and penalty calculation. I am not as familiar with TT Online but this form may be in the PDF with "All forms and worksheets"
The issue you are finding is the penalty is determined quarterly so you could meet the safe harbor amount (100% of 2024 or 90% of 2025 whichever smaller etc) - or even overpay and have a refund - but on a quarterly basis if you paid estimated tax unevenly it could generate a penalty for the quarters earlier in the year (with 8% penalty rate). The assumption is the tax you owe is earned evenly thru the year, and that safe harbor total amount is divided by 4 into quarters, and if your estimated tax doesn't line up with that either in amounts or payment dates (generally referred to the IRS as "timely"), it will generate a penalty.
If you paid unevenly due to the fact the income was uneven e.g. you had an unplanned Roth conversion or large cap gain event late in the year, then you can adopt the Form 2210 "Annualized Income" method to show your AGI etc by quarter to line up with the estimated taxes. You have to provide cumulative AGI, Cap Gains, Qualified Divs etc for 3/31, 5/31, 8/31. Maybe similar for state. This can be a lot of work and may not entirely eliminate your penalty. In TT you can go thru this process in "Other Tax Situations" under Underpayment Penalty section.
For 2025 planning under "Other Tax Situations" "Form W4 and Estimated Taxes" you can provide 2025 estimates for your income to determine whether to use the 100% of 2024 or 90% of 2025 to pay estimated taxes. Unless your 2024 income was unusually high for some reason then the simplest approach is to pay 100% of 2024 in even estimated tax payments since everything is known and you should not get a penalty even if you have some large unplanned income event at any point in 2025.
More on Estimated Taxes here https://www.irs.gov/faqs/estimated-tax
Form 2210 instructions here https://www.irs.gov/pub/irs-pdf/i2210.pdf
Not a CPA/Expert just my 2 cents having gone thru the AI process for 2024 and keen to avoid doing it again - hope this helps.
