Look at what offset 0x608 points to after anyone runs an ADD_KNOWN_SPELL - it's not the number of stars for that proficiency anymore (known spells come first after the .cre header, adding them in moves the effects array further down the file). It's likely to end up in one of the other effects, though if there's many spells it'll end up in the spell memorization table (bad!)
Write the code however you like it, of course - I'm just suggesting that you make sure you're writing to a weapon proficiency rather than crossing your fingers and hoping you write to the correct part of the file


As long as I'm making a nuisiance of myself and bothering you, COPY_EXISTING ~file~ ~override~ is fine, COPY_EXISTING ~file~ ~override/file~ is fine if you need %DEST_RES%, and COPY_EXISTING ~file~ ~override\file~ means your mod doesn't install correctly on the Macintosh.