To be clear, the machined face of the timing cover (perpendicular) and the actual pan rails ARE the shaft angle. The problem with the "shaft angle" is as I started to say........you can't measure it absolutely accurately when it's slopping around loose at the rear of the trans.
OP what you want, in the end, is for the axle pinion shaft and the crank centerline (which again IS the transmission shaft) to be PARALLEL when the car is under power. You have to use a bit of "cheating" to allow for spring wrapup and suspension movement. Most people set the axle a couple degrees below the crank centerline to start, and it should "wrap up" under hard pull
Measure this with weight on all 4 wheels, level surface, and proper weight in the car, engine trans, spare, gas, etc. Even a driver if you can muster that
