Let the sailmaker worry about the right amount of luff curve and prebend of the mast. In principle is the luff curve always more then the prebend ad you want a flat sail (when layed down on the floor) to be a curved wingsection when it is flown on your mast. To force the flat (two dimensioanal) sail into this 3 dimensional shape you need a difference between the sails luff curve and the bending of the mast. You just don't want to much of it, depending on conditions and crewweight so you reduce this difference by prebending the mast. There are some more complex working added to this as well but thwn we'll go too much into detail. If you are interested in that then try to get a book on saildesign and read it. It will be far too much for me to type it all up. It is also a subject that is best learned and understood when you are actually seeing its effects on your boat happening. So my basic advice is to have the sailmaker do the design work of the sail for you after you have just specified you intend crewweight, usage and dominant sailing conditions.

Those 20 mm I'm running was a experiment after I had a sailmaker design and build me a mainsail. I think however that I'm running a different prebend right now, one that suits my way of sailing and conditions better. I can't remember how much I'm using right now, but I can look it up in my trim and tuning booklet.

Wouter