I'll jump in here even if you adressed Wouter..


Pre-bend is how much the mast has been bent out of column by sweeping the spreaders aft and pulling tension on the diamond wires.
The sail have luff curve cut into it, not pre-bend

To determine the amount of luff curve, you ned to consider several other factors as well as pre-bend. Your sailmaker uses luff curve to put depth into your mainsail. If you put your main on a flat surface you will notice that it lies mostly flat. But when you hoist it on your mast it has draft. The reason is that the luff curve is quite a bit more rounded than your pre-bend, so the extra cloth 'transforms' into draft.
To determine luff-curve, your sailmaker not only needs to know how much pre-bend you use, but also how stiff your mast is and what bending properties it has. Your bodyweight and average sailing conditions also goes into the equation. When your sailmaker have a good knowlegde of all this, he will design your luff curve according to all these factors including how much draft you will need. He will rarely get it right the first time he builds a sail for a new type of boat/mast, hence it's a good idea to use sailmakers with experience on the specific masts/boats.
To further complicate the matter, you should tune mast pre-bend to your body weight and mast stiffness (no two alu masts are alike) and have the luff curve cut accordingly. Only then is your mast pre-bend and luff curve matched and your power-plant able to give you 100%. While the Tornado had alu masts olympic crews was constantly looking for masts with optimum bending characteristica for their style and crew weight. Usually they lugged around two or three extra masts on their trailing rigs, and breaking their favourite masts was a minor disaster.
Consider the last point as trivia unless you are on an olympic/worlds skill level.

If you want to look at this for yourself it's an eye opener to bring a digital camera along and hoist your main. Sheet it in a bit and take a picture from 10-15 meters away abeam of the boat. Then sheet the main max, max the downhaul and do another picture. Go home and put the two over each other in photoshop/gimp and look at how the mast curve/bend have changed. Rotating the mast will change the bending properties further. This is a fun exercise, enjoy