If the plate is run by a synchronous AC motor, its rotational speed will be highly stable and accurate. You could probably get by with a little bump like a screw head or something on the bottom of the plate to operate a microswitch once per rotation. Then you can use millis() and compute the table position as a function of time since last switch transition. I think you could get within 1/2 degree, as long as the table is still plugged in and moving all the time.