If he's last minute dude, don't ruin your Saturday.  Guess high, or just let him tell you what he wants to pay.

AND - his first estimate is due Tuesday also.  IF he cares.

Word of advice, I usually include the first estimate amount with the extension payment with the anticipation of the overpayment being applied.  That way, if the guestimate is wrong, 2024 is covered and only the 1st qtr 2025 estimate is (possibly) low.