@Bookwoman the IRS does not require form 2210 unless one or more boxes on part II is checked. try box B and then carefully review the numbers if the IRS provided a calculation check the Turbotax 2210 vs the IRS.
as noted by others withholding is assumed to be 25% of the annual total for each period and estimates must be paid on or before the due date. if your withholding did not occur evenly throughout the year you can use the actual withholding for each period also check box D on the 2210.
4/15 withholding 1/1-4/15
6/15 withholding 4/16-6/15
9/15 withholding 6/16-9/15
12/31 withholding 9/16-12/31
if you come out with a lower penalty even zero send the 2210 and IRS notice back disputing the IRS penalty.
if you made estimated tax payments by mail the IRS usually goes by the postmark date and I think then moves it back a few days. for example, if you put it in the mail on 4/15 but the USPS doesn't get it until 4/17. that would be backdated to 4/15.
