Sorry, what I should have said was the surface where you are placing the magnetic degree indicator on. I know alot of people that mess this process up and get inaccurate readings because they measure off the outer case of the rear end, or transmission, vs the actual yoke/spline itself.
Example…rear end.
-Drop the drive shaft and remove the u joint clamps from the yoke completely
-Wire brush and scrape clean the end face of the yoke where the joint band clamps bolt into the yoke.
-Using a clean flat piece of steel plate(or better a machinist granite) place the plate against the end face of the yoke and put the degree indicator up against that to get your angle measurement. That will get you the true reading of the yoke angle, not the rearend case angle which may or may not be exactly the angle of the yoke.