You can't really know what the angle is to the ground, unless the car is on jack stands sitting on a chassis plate, or you do the math to see what the distance is from the front to the ground, and the rear to the ground.
The angle of the ground the car is sitting on, the difference i front and rear ride height, and the different in front and rear tire diameter will all affect a degree reading if compared to the ground. For instance, your garage floor slopes back-to-front, so any fumes will roll out the garage door. If you've got the car lowered two inches in the front, and it's backed into the garage, measuring the pinion angle in relation to the ground is going to have a different reading than if you pulled the car in forward and took the reading.
Measure it from the tailshaft of the trans, and the pinion.
Now, what that measurement is supposed to be depends on how many different people you ask. I've always heard 3-degrees down pinion, as the rear will point slightly up under acceleration and drive.