I was taught to get the two as parallel as possible and then add one negative degree to the differential side so that spring wrap-up (which will happen) won't put your differential angle less than your transmission angle. Oh, and measure on machined surfaces. That hasn't been mentioned yet, but I think it ought to be. I did this not very long ago on a 3100 with everything new...springs, mounts/shackles, everything new and all stock height. The transmission angle was +3 degrees and a hair, so the differential was set at -4 degrees on the nose. Good luck. As Joe said, this can be a can of worms.

~ Jon
1952 1/2 ton with 1959 235 | T5 with 3.07 rear end