Adjusting the slipper on the 501 should be as simple as tightening or loosening the nut on the layshaft on the spring side. tighter the nut the less the slip and vice versa.
I've never heard of adjusting a slipper by putting a driver in the grub screw of the pinion and rotating the wheels. Can't see that doing anything other than possibly testing the front or rear diff to see if it will slip... I'd ignore that method if I were you mate.
