I believe that Paul had the right explaination of the 90 degree rule. It is the angle between the upper arm and lower arm. That said, the rule applies only for a forced prone position shot. If you shoot prone on the non-forced position or freestyle position shots, you can shoot anyway you want so long as you support the rifle soley with your body and your body is not supported by any other object or the ground. The hand cannont rest on the bum bag, ground, or anything else per rule F in the Shooting Position section.