You have some room on engine angle I don't believe there is one exact answer.
What is important is driveline angle, very little to no room for error there.
You can adjust the pinion angle but should also consider engine angle while you are installing it.
Try this tech tip
http://www.stovebolt.com/techtips/driveline/setup/index.html

Grigg

•1951 GMC 250 in the Project Journals
•1948 Chevrolet 6400 - Detroit Diesel 4-53T - Roadranger 10 speed overdrive - 4 wheel disc brakes
•1952 Chevrolet 3800 pickup
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"First, get a clear notion of what you desire to accomplish, and then in all probability you will succeed in doing it..." -Henry Maudslay-