There are huge differences in sails, especially brand to brand. A lot of the Euro sails tend to use straighter luff curves which may require less prebend. Masts vary and sails change over time.

Spreader rake in combo with tension change the prebend. It has to be smooth and it has to match the sail. Note the diamonds only effectively change about ½ to 2/3 of the mast shape. Knowing your settings for down haul and rotation to properly set the main sail is even more important to maintaining power. Any discontinuity will detract from the sails performance. Having too “fat” a sail, from improper prebend or rotation both will make the boat feel very sluggish and detract from your ability to point and power up in light wind. Beware as I have seen a lot of people feel they have no power in light wind and arbitrarily adjust things that make the problem even worse.

On our web site under support we have a tuning guide which describes what you should be looking for in setting up your mast. That being said I have never seen a sail needing 125mm of rake, that sounds a bit excessive, but to arbitrarily say you need 50 or 75 would likewise not necessarily provide you with a setting that works on your boat.