late payment penalty applies since it was due 7/15/2020. the amount doesn't matter. the $1,000 your thinking about is for underpayment of estimated tax - no penalty if the rules are met but any balance due even under $1,000 was due 7/15/2020.
there is a late payment penalty of 1/2 % per month or fraction thereof + the interest. let the IRS bill you. generally, nobody gets it right but send in $512 and then the IRS should leave you alone. assuming the rate is correct you are doing it wrong
+3.25%/365 = daily rate compounded for 92 days (to 10/15/2020). (would still come out to about $4
