I agree with DR Diff. Look at this link I posted as the first few paragraphs in the orange shows and tells you about pinion angle very well. The rear is always lower then the trans on a stock muscle car. What I like to do is set the trans centerline at 0 first if it is not. Then roll the rear to 0 and then move it down from 2 to 7 degrees depending on whay you use the car for. The reason the rear is always lower then the trans is as Dr Diff said you dont want the rear and trans running the same paralell line pointing at each other. The drive shaft has to have some angle so the U-joints will lube. You just want the trans centerline and the pinion centerline the same as paralell to each other but the pionon centerline lower then the trans centerline. Once they are paralell just roll the pinion the degrees down you want. Ron

http://www.moparts.org/Tech/Archive/axle/8.html