I'll throw in 10 cents here.
If the breeze is up, say 12 apparent or more, partial backwind of the main is a go fast trim condition and not a concern. There is too much wind coming thru the slot not to affect the main.
I lighter winds, if your main is being backwinded, somenthing is off. Assuming the genoa turning block is in the right location
- Is the boom on centerline.
- Is the genoa draft at 50% or further forward.
- Is the jib
sheet eased several inches
Thos are my guesses.