Yeah, i think your best bet for accuracy and repeatability is an encoder, not a potentiometer. Get an encoder (optical or magnetic) and put it on the worm gear's shaft, then determine how many rotations of the worm gear give you a rotation of the main shaft. The encoder on the worm gear shaft will probably only need a few pulses per rotation for you to get 1 degree of accuracy on your main shaft, and since you have a fixed number of pulses per rotation of your encoder and a fixed mechanical linkage, you get very little drift or accruing offset.