RE: Formula Sail Sizes?
I have an 8.5 Neutron which i rig on either a cut down 520 32 or a 490 29 depending upon the windstrength. The stiffer mast gives me a tighter leech in relatively lighter winds. The Neutron seems to cope well with either. I guess a 9 would be OK on a 520 so long as it is not too stiff.