Pete,
The discussion about spreader rake and all of that could take a board of it's own, especially with all of the varying opinions on this board.

So,
Here are some links that can get you going.....

http://www.ahpc.com.au/PDF/VIPER%20Tuning%20Guide.pdf

That says Viper tuning, but there is some explanation of what spreader rake can and do. So, that would pertain to all boats, even monos....

another.

http://www.landenberger-sailing.com/spip.php?article21

That one takes reading a couple of times to get it, then you need to experiment with it.

The Viper tuning also explains how to measure the spreader rake.

What everyone needs to take in consideration that the bend characteristics of all masts and sails will be different.

Basically,
If boat 1 and 2 have a brand new main from the same manufacturer and the masts are the same(wing masts), the bend characteristics of mast 1 could be different from mast 2. In other words, what rake works on boat 1 might not work on boat 2. But, discussing it with another sailor should start getting you in the ball park.

The ONLY way to REALLY know, go out and sail against another boat.

Mike.