Spreader rake is a method of tuning the fore-aft stiffness of a mast below the hound fitting. Crew weight and sail cut determines the required amount of spreader rake, as it has a significant effect on the amount power developed by the rig.
Determining your correct rake is only done by sailing the boat and knowing what to look for.
If you have excellent height up wind, but lack boat speed and the boat won’t accelerate in the wind gusts, then you need more rake. This helps the mast bend fore and aft, which, in turn, allows the sail to flatten and the leech to open in the wind gusts. (Also see notes on diamond tension in tuning section).
If you are lacking height and "grunt" in light to medium weather, then you have too much spreader rake.
The spreader rake is measured by placing a straight edge or string-line between the diamond wires at the spreaders and measure the distance to the back of the mast.
Settings depend heavily on crew weight; with heavier crews generally carrying less rake than lighter crews.