@kleinpaul I am glad you asked the followup question. To qualify under the 110% exception the only payments that count towards the 110% are withholding on a W2, withholding on a 1099 or withholding on some other form, plus excess social security withholdings. You can see this at the top of page 3 on this document: 2021 Instructions for Form 2210 (irs.gov) (see instructions for Line 6).
To avoid underpayment penalties by making estimated payments at irs.gov (or by other methods like EFTPS or mailing checks) you need to pay in enough to get to a balance due of less than $1,000.
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