Follow these steps:

Place the bike on a stand and extend forks and shock completely. Then measure “vertically” from the axle bolt
to a reference point on the fender. You can tilt the tape measure slightly towards the front of the bike to allow for arc-motion in shock travel, Make sure to use same reference points when measuring.
(Ex: 20”) This is A”

Take bike off the stand; have the rider step onto the bike (without bouncing) then in the standing position. If needed, get a person to balance the bike or have the rider hold onto something. When the bike is settled, measure the distance between the axle and your reference point on the fender.
(Ex: 17”) This is B”

Subtract B” from A” (20”–17”) = 3” RACE SAG

To check Static-Sag, Immediately after the rider has stepped off the bike, measure from axle to your reference point on the fender.
(Ex: 19”) This is C”

Subtract C” from A” (20”–19”) = 1” STATIC SAG

Note:
Tighten preload collars and pitch bolts.
Check sag every 4-5 races.