I have read centerline's pinion angle theory on the bulletin board on two different threads. I have read the same theory on other sites as well. The diagrams he uses is concentrated on drive shaft angle. It's called pinion angle for a reason. You measure the angle of the pinion. As you add weight to the vehicle. A trailer for example. The angle of the driveshaft will change. The rear of the vehicle will sag causing the driveshaft angle to decrease. But the pinion angle is set by the leaf spring saddles or four bars, etc. No matter how much weight is added to the vehicle the pinion angle never changes because it is centered on the suspension. However, you do set pinion angle with the full weight of the vehicle. Jacking the car up will alter the magnetic protractor's reading because the whole vehicle is now on a downward angle.
The instructions from South Side Machine for setting pinion angle can be found on the http://maliburacing.com/