Your pinion angle is set by your driveline angle (engine/trans angle). Your pinion should be parallel under typical load, so if the trans output is -4°, the pinion should be +4° to be parallel with it. Since this is a leaf-spring application, the pinion angle will change under load, and typical is to lower the pinion -1 to 2° for street cars, and -2 to 4° for drag cars so it is parallel under typical load, but your results may vary due to a bunch of factors.
David