I decided not to include a penalty but to include the following explanation of waiver of penalty (with actual amounts, not XXX, etc.):
I paid more than 110% of my 2020 tax liability in estimated taxes. My total tax last year was $ XXX. TurboTax says the maximum required annual payment based on prior year's tax is $110%XXX. The total of my estimated tax payments for 2021 was $124%XXX. (My underpayment was owing to a surprise sale in December of real estate. I increased my next estimated payment and was hoping to supplement that with another estimated payment but was not allowed to do so because I unknowingly missed the January 15th deadline.)
