Car MUST be "on wheels" loaded IE gas fuel, spare whatever is "normal" driving loads.
There is room for "opinion" but the SIMPLE answer is that the CRANK CENTERLINE and the PINION SHAFT must be PARALLEL under full acceleration.
What this means is, that due to the flex of the leaf springs, there is some "flex" between "unloaded" (torque) and full throttle, and THAT is where you get the discussion about whether it should be 2* down, etc. The fact is it depends on spring stiffness, and "experience of others"
While you are at it, make sure the shaft is "built" correctly. "u joint phasing". The simple answer for these one piece shafts is that you should be able to look down the shaft like a rifle, and the end caps on both ends should be in perfect alignment. if not, the shaft is either twisted, or welded up incorrectly
