There are 2 different sets of angles that need to be addressed - PINION ANGLE and U-JOINT OPERATING ANGLE . Pinion angle is the relationship between the engine/trans centerline and pinion gear and addresses vibration. U-joint operating angle is the angle between the engine/trans centerline to the driveshaft in the front and the pinion gear to the driveshaft in the rear, and addresses wear (until it gets over 10*, then CAN start to cause vibration). While the Rossler chart is helpful in showing the relationships between the individual parts (1) it isn't intended to be a guide for leaf spring cars (2) it is technicially incorrect since it doesn't address u-joint operating angle. In other words, you can have your centerlines parallel, but if you don't have any, or you have too much operating angle, IT'S STILL WRONG! You may actually have to adjust your engine/trans centerline to get the desired 1-2* OPERATING ANGLE on the front u-joint, then set the PINION ANGLE after that.

Now, all that being said, leaf spring cars are a crapshoot on how much pinion angle they need, since the amount of spring wrap-up varies greatly. Low horsepower-poor traction (tires) probably only need 3* pinion angle; high H/P, high tracton cars may need 5-7* pinion angle so that the angle is correct under power. But if you are deflecting the springs that much you probably should be looking at ways to control spring wrap-up.

And lastly, again I see another mention of pinion angle changing or not changing 60' times - I've challenged many times for someone to show me in mechanical theory how pinion angle changes traction - IT DOES NOT . The means you use to change pinion angle changes other things that may change traction - I/C, ride height, wheelbase, etc.