Shock "free" length & "compressed length is very important. I think that we have a video on our web site houser-racing.com that shows how to get this.
First determine the "compressed" length. This is where the ATV will be when it bottoms out.
PUT THE WHEELS & TIRES on that you will be using. Changing tire diameter by 2" changes all this below by 1"! Very important.
Find Compressed Length.
Take shocks off ATV. Let the ATV sit 1.5" off the floor. Most people put 2x4 flat under the frame. On a big jumps, at least for MX racing, the ground will flatten the bottom of the tires by 1.5" & the frame will drag the ground at this point. Measure eye to eye distance. This is your compressed length. Do you see how tire diameter would change this? If you put 2" larger diameter tires on, the ATV sits higher 1"
Find Free Length
If you want 10.5" of wheel travel, that is what is used for MX & can change math for your desired wheel travel, add that distance to the distance under the frame; 1.5" + 10.5" = 12" under the frame. Measure eye to eye & this is your free length.
For the pros, they always get their shocks made after this exercise.
The pros may fine tune this alittle. In XC we have found that the guys do better when the rear end of the quad can go compressed with .5" under the frame & then they use 11" to 11.5" of rear wheel travel.
Hope this helps.