It's a funny one Stanley, we set a guide based upon the ISAF rules
www.internationalwindsurfing.com/files/usercontent/pages/Sailnumbers_for_windsurfing_racing_classes2.pdf but when you look at the PWA sail numbers they are all over the shop.
I would say that your trouble comes about because you are trying to put the number on what would be considered a fairly small sail for a race sail. I would probably go with the AUS above the Number or try putting the number on a larger panel lower on the sail. For local events you just need to be easily recognisable against your peers at a distance.