When I did my new rear end housing, I used Calvert spring perches and welded them on to have a negative 3.5* compared to the output shaft of the transmission. On the old stock housing I had to use angled shims on the perches to drop the pinion down enough to work correctly. I would say if you are running a drag car -4 degrees is good, if more of a street car then -2. I have heard of some guys going to -6 or more, but that will kill your U-joints eventually.
Remember, the ground or the drive line angles mean nothing, measure with the driveshaft out, and it is the total number that counts, so if the trans is at -2 then the pinion should be -2 for -4 total. or if the trans is at 0 then the pinion should be -4. I used a digital angle finder with a magnet base, but the dial type work fine also. Under hard acceleration the rear pivots up 3-4 degrees so both shafts are parallel.
Put some preload on your caltrac bars, maybe just 1/3 to 1/2 turn, then adjust one or the other to make a straight launch if it goes to one side. If it goes to the right, put more preload on the right bar to straighten it out, etc.