Brian, there is no real "black art" to designing and making sails, I know a lot of sailors that have successfully made (and some still make) their own sails, it just seems that the majority of sailors either "can't be bothered and will pay the price" or think that it is all just to hard. If you purchase one of the books by C.A.Marchaj "sailing theory and practice, in it you will find all the reasons how and why a sail performs and once there is an understanding of that, it's then quite a simple matter of applying those principals to the relative size/shape of the sail plan that you need and draw out. Once youv'e done that, any man/woman who can use a sewing machine is more than capable of stitching a one off sail together (even on an electric "home" sewing machine). From actually doing it quite a few times personally, I can tell you that, it is a LOT cheaper than a sail maker, and that the hardest part of it was actually convincing myself to start the first cut. After the first one, the rest are easy.
You know that before the introduction of Mylor and Kevlar etc, into sail making, many top sailors in class's around the world would either make their own sails or make then in a sail loft with the help of a "professional" who would be only to pleased to assist them as the professional would use that cut for the "class" if it was successul. Most sail makers realise the the people who know their class sail performence best are the top sailors in that class and use their knowledge instead of visa versa.
Darryl J Barrett