In general, sag on your rear shock should be set to 15 - 25% of shock travel, depending on riding conditions or personal preferences.
To measure sag on your rear shock:
Before sitting on the bicycle, measure and record
the distance from the center of one shock mounting bolt to the center
of the other shock mounting bolt. This is known as the "eye-to-eye"
Air shocks have an O-ring on the shock body. The O-ring should be pushed up against the scraper lip of the air sleeve without the rider on the bike. If there is no o-ring, use the "eye-to-eye" method.
Sit on the bicycle in a normal riding position. Your weight should be distributed on the saddle, handlebars and pedals. It is also recommended that you are properly outfitted in your riding gear. It may be necessary to hold yourself up against a wall or post to steady yourself. Do not bounce on the pedals or saddle.
Have an assistant measure and record the eye-to-eye
For an air shock, dismount the bicycle and measure from the scraper lip to the O-ring. This measurement is SAG.
#2 from MEASUREMENT
#1. The difference is SAG.
MEASUREMENT #2 = SAG
(E.G., 7.875 – 7.275 = 0.600)
Consult the air or coil springs settings table in your respective shocks' section. If the sag specification is not in compliance, follow the instructions in Setting Sag in your respective shock's section.