what pinion angle?

Put the car on stands or on a drive on lift....

1. measure the angle of the driveshaft installed in the car...
2. measure the angle of the pinion or across end yoke

3. set the diff 4.5 degrees opposite the driveshafts angle.

Now when you are under hard load, the rear angle should be 0 or .5 degrees and you won't have additional loss thru angle.