I just happened to write this for another forum I'm on.
The static sag will only tell you if the springs preload is in spec from it's original condition.
There is a certain order to do this.
Step 1: put bike on stand and take a measurement between the axle nut and a fixed point like the rear fender or side plate.
Step 2: Take the bike off the stand and sit on it wearing all of you riding apparel and now measure it. (it helps if someone holds the bike while you sit on it so all of your weight is on it).
The difference between steps 1 and 2 is the race sag.
Once you get 100mm difference move on to step three.
Step 3: Take a third measurement while the bike is on the ground all by itself from the same place you took it from the previous steps.
The difference in measurements between steps 1 and 3 is the static sag.
If after you get the desired race sag and the static sag is too low then you probably had to crank down on the spring adjusting nut alot to get it there. This means that the spring is either out of spec or you are to heavy for that spring. The same goes in the other direction. You want a spring that can accomplish the correct race sag AND static sag so that the bike rides in the proper range of the stroke most of the time. I don't usually bother with the static number as most of the manufacturers make the springs for my weight, I'm 170 lbs so I always just look for the 100-110mm range. I also like to change the sag depending on the track and conditions. More sag=better straight line stability but loses cornering ability. Less sag=better cornering ability and a little more resistance to bottoming but jacks up the rear end a little and can kick around a bit on bumps.
And since I do ride an open bike...here is the tire I use
http://www.mandhtires.com/store.php?crn=68&rn=276&action=show_detail