If you want to do it right, get a fat friend or remove the rear shock. Get the swingarm parallel to the ground, and set the chain so it has a very small amount of slack(not tight or loose). Lock the axle in place, and remove the fat friend or replace the shock. Now push the chain straight up from the back of the chain slider and measure the distance between chain and slider. Write the measurment in the "notes" section of your manual.