I don’t quite get your question. However, a significant underpayment is calculated quarterly. For example, if you owed $8000 when you filed your return on April 15, 2021, you would have $2000 back due to the spring quarterly payment, $2000 back due to the summer quarterly payment, $2000 back due to the fall payment and $2000 back due to the winter payment. This year, both the spring and summer payments are due July 15, with the fall and winter payments due September 15 and January 15 as usual.
If you made no estimated payments you would use the short method on form 2210 and your penalty would be 3.398% of the amount you owe. If you made some estimated payments, you would calculate by quarter to determine the actual penalty amount.
