Pinion angle is the difference between the centerline of the transmission output shaft and the centerline of the pinion gear UNDER LOAD. Meaning when you're accelerating they should be parallel. This does not mean pointed at one another. It means parallel.
As an example or a typical drag car...If your trans output is pointed down from parallel to the ground say 4°(big block and motor plate) and the pinion as measured is 2° down from parallel with the ground(SS springs w/ the shorter from segment), that means you need to shim the leaf spring perches or cut and relocate them to get the pinion down far enough to get proper pinion angle under load. In this example's case that would be 2°. Leaving the pinion "down" 4° from the trans output at rest.