There is no hard and fast rule as there is so many variables which come into play, there are some guidelines which help, but nothing goes past test and tune experimentation

There are plenty of guidelines, but they all depend on what gear you're using, and what you're trying to do with it...

Almost all of the newer generation of shortboards will be happy with somewhere between 132 and 135cm from the tail.

Have a look at some of Cribby's stuff: www.guycribb.com/windsurfing_technique_holiday_DVD_0076v01.htm

Some people are firm believers in his methods (especially the harness line and boom height part), and others say it is complete crap, it's up to you to try it and decide for yourself!

The biggest problem is that it's all interrelated - change one thing, and you'll probably have to alter something else as well..

Elmo is right, tune, test, tune, test.. It's really the only way!