As Tom stated, you have to set the pre-bend in the mast to match the particular sail you have. There are many combinations of rake, tension etc that in the end will all do the same thing depending on your sailing style and conditions.
When you set up a mast (any mast I guess) to the mainsail you have you are trying, somewhat to match the luff curve that is built into the sail right? But if you create so much prebend to match that the sail will be seriously flat (depowered). There is a between that you set the prebend to then. Would a decent idea be to start with some prebend then completely match the luff curve of the sail when completely downhauled? I am just theorizing. I think we all (most of us maybe) talk more than we sail unfortunately.
