Thanks for answering!
Working through all the possibilities it looks like TT is calculating box 4 100/110% of 2020 Taxes on the Estimated Tax Payment Options worksheet using 1040 Line 24 Total Tax minus 1040 Line 30 Recovery Rebate Credit (and any other credits that would be totaled on Line 32). So the TT 2021 1040 ES calculated amount appears correct.
