Adding Pinion Angle Shims - Tell Me If This Sounds Correct, Please

Whatever the trans angle measures, down 3*, you want the pinion UP 3* for prallel state, then roll it down to obtain the "roll up" variance. Your math approach looks correct.
^^^ This! Don't overthink it. Make the pinion parallel to the trans and rotate it down 2-3*. Yours is good. Run it!