Correct way to set rear angle?
What suspension are you running and do you plan on driving this on the street a lot?
Looks like a leaf, SS spring?
If driving on the street try to go no more than 3-4* down. Any more than that and the U-joints get a real beating.
Track only, 6* down would be OK.
Street driven car, I'd limit the pinion to level or 1* up.