Pinion angle and driveshaft length.

I guess im confused here... From my readings SS spring cars like 5* down or more. Why would I set it to 0*. Maybe Im figuring 2 different things here. The trans is at 2* down. Is one reading just to measure driveshaft length then the other is to actually set pinion angle?
Yes setting at 0* is for driveshaft measurement(length).When I set my Dana60 up,I set the pinion 4* down.FYI Mr Crackedback helped with advise back then.Works for me.:D