Ideally, you want the pinion to become parallel to the transmission when the vehicle is under power. This works regardless of weather the transmission is mounted higher OR LOWER than the rearend.

For example, if the front joint is lower than the rear joint, and the transmission slopes 5 degrees downward toward the rear and the pinion slopes 2 degrees upward toward the front, the result is a 3 degree negative pinon angle. In other words, the pinion must rotate upward over 3 degrees before the angle goes positive.