This formula does increment the letter, but how to make it re-start based on a condition in col A?
=A3&"-"&CHAR(ROWS($1:1)+64)
Here is a test sheet: https://docs.google.com/spreadsheets/d/1gh5w0czg2JuoA3i5wPu8_eOpC4Q4TXIRhmUrg53nKMU/edit?usp=sharing