As mentioned above, you are mostly trying to find a good compromise with comfort (elbow), weight distribution, and clearance with the motor. Once I find that spot on the rails, I try not to move it much. I have 3 different length chains for my kart, 54-56-58 links. I use different size chains with different gear ratio’s, basically to help keep the motor from moving more than 3/8″ either way. I have a chart of the gear ratio’s I have available in my tool box. For example, if I go to a 16-26 ratio, I know a 58 link chain will keep the motor and the shift lever in the same spot. Makes gear ratio changes quick and easy in the pits.