Stand on the footpegs as it gives a more consistent measurement than sitting on the seat. Bounce lightly on the bike to overcome stiction and measure the compressed distance. The difference between the two is the rider sag. The correct amount is approximately 33% of the bikes total available travel, usually between 95-115mm on full size bikes. The correct amount varies because of bike geometry, rider preference, and type of riding. The rider sag should be set with the rider in full gear. The bike should have correct levels of fluids, be free of excess mud, and the shock should be cool to the touch as heat build up changes sag levels.