Clocking a case 10* gets the bottom end of your T-case level with the bottom of the tranny. Which would normally be considered flat. Why do you need 20* or 30*. What is that going to get you?
Most of the time if someone wants a flat belly, clock the case and then raise the tranny. But clocking the T-case any more than that doesn't get you anything but a really bad pinion angle.
