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?
Wow I'm lost here. I guess when I get to this point on mine I'll be asking all these guestions all over again. lol !