The White power book that came with your bike is your best bet. Static sag should be 35mm, the difference between when the bike is on a stand and then standing on the floor. After that the rider sag should be 100mm, the difference between the bike on the stand and the bike with the rider on it. The conversion of inches is 1" =25.4mm. Or try http://www.sciencemadesimple.net/EASYlength.html
The sag on my 2001 400MXC was way off. I had to remove the shock to loosen the locking screw and adjust the preload. It's easy to do, support the bike under the engine with a crate or ?, remove the upper and lower shock bolts and slide the shock out. Use the wrench that came in the tool kit to tighten or loosen the spring, put the shock back in and check the sag. Make sure to re-tighten the locking screw.