Cass (Dr Diff has it right)....pinion angle is in relation to the tranny output shaft. for a street car, measure the output shaft of the tranny. then measure the pinion. make them parallel, then roll the pinion down about 2 degrees, so they're parallel when the car is under power. driveshaft angle is incidental, as long as it's not in line with the tranny output or pinion.

so if the tranny is say, 3 degrees nose down to the world, the pinion needs to be 3 degrees nose up to the world to be parallel. Then it needs to be rolled down about 2 degrees, so the pinion is 1 degree nose up to the world, so it's 2 degrees nose down to the tranny.