The best work available is "The art and science of sails" by Tom Whidden. It is not aimed at beachcats, but it is the best there is. There are very few books about saildesign available. If you want to learn about saildesign you are more or less forced to work with an sailmaker or go trough the trial and error route. "Maximum sail power" is also relatively good if you can not get Tom Whiddens book.
About luff curve. There are two ways of giving a sail draft. Either by using broadseaming between panels or by adding cloth to the luff. You can distinguish between the two by laying the sail on a level surface. If the sail lies flat, cloth is added to the luff. When the sail is hoisted the extra cloth has to go somewhere which gives the sail draft. If you measure the luff curve of such a sail and enter it into Sailcut, you are giving the sail too much luff curve! The consequence is that the sail will have too much draft and it will be slow with poor pointing.
The luff curve on the Tornado mainsail design (which btw. is based on a Marcus Towell design) is pretty much correct. When I adapted it to our weight and conditions I gave it a little bit too much luff curve, so we had to compensate with increased diamond tension.
Luff curve is a direct function of how your mast bend, which again is a direct function of materials, profile, spreader rake, diamonds tension, boat, crew weight and predominant sailing conditions. With a composite tip the upper part of the mast will bend more than the lower section. We know that square top mains require higher sheet tension, so this would need to be compensated for in the design. I have no experience with composite tips, so I dont know if the idea of a square top is out of the question on such a mast.
Sailcut CAD works with a mathemathical model of the sail you design. So if you enter values for luff, leech and gaff which dont match up Sailcut CAD will let you know by setting the misbehaving value to the default. If you are unable to make the gaff angle larger than 90deg, try adding to the leech length.
Sailcloth can be bought directly from the manufacturer (Bainbrigde, Contender e.a.), just dont let them know you dont run a company.
