Compare the parallelism between the transmission and the pinion.
A zero degree pinion angle is when the transmission is parallel with the pinion.
A negative pinion angle is the number of degrees the pinion must rotate UPWARD to become parallel with the transmission.
A positive pinion angle is the number of degrees the pinion must rotate DOWNWARD to become parallel with the transmission.
For example, if the transmission slopes downward 3 degrees toward the rear, and the rearend is level, you have a true 3 degree negative pinion angle.