Do you have the air spring or coil spring version of the fork? Performance Bike has a generic guide for setting the preload/sag, rebound damping, and compression damping of a suspension fork. Your fork has 130mm of travel. You'll want to aim for a sag of around 25-30%, so 32.5-39mm or 1.25-1.5 inches. If your fork has an air spring, you'll need a shock pump ($20-25) in order to be able to set the preload. If you have a coil spring, I don't know what you do; I've only ever owned forks with air springs!