The ground has nothing to do with pinion angle unless your output shaft is level with the ground. You can stand the car on end you still make the trans output shaft parallel with the pinion shaft . Then tilt the pinion opposite of its travel during acceleration a certain degree depending on your calculated twist of the differential . That calculation changes with suspension which controls torque wrap of the rear.
It's pretty easy , Something a kindergartener can do. You all are making it into a geometry class.
Make the shafts parallel and tilt the nose down so they are parallel when you hit the throttle. Put 2 socket swivels on three extensions and play with them . Not that hard to see how it works. If the end 2 are not parallel you won't turn them without bind
