if you are on desktop go to Forms mode, open Form 2210 (by default it may not be shown in your return forms lists until you select the AI method), and it will show the safe harbor calculation and allocation by quarter to see where the underpayment arose earlier in the year, there should be a form with the penalty calculation also. Not sure equivalent for online but it should be in the PDF with all forms and worksheets.
if you start out with intent to cover 100% of your tax but end up having to adjust ES higher in later quarters to meet the safe harbor, then you'll end up with an underpayment from earlier quarters (even if you overpay and end up with a refund).
if your income is steady it may be simpler to pay estimated tax based on 100/110% of prior year tax which is fixed amount but could be an overpayment especially if you're in the 110% category.
