My CPL QXC was 350nm in the logbook, but straight line would have been really quite significantly less.
This was accepted by the UK CAA, and I flew the same route as the other 11 guys in my class, and this route was the same as the hundreds that have gone through the same training school after me.
Personally, I would photocopy your map and PLOG, providing a legal document that proves that your flight was over 300nm and don't stress over it.