The top figure shows the movement. Tool on top ( dark colour disc). The bottom figure shows amount of glass remained as dark bars 1/6 D CoC 1/4 D CoC 1/3 D CoC 1/2 D CoC 2/3 D CoC 3/4 D CoC optimal setting. As these are simulations, and if some polishing ( grinding ) process is done by human beings, needs to factor in the man-made variations in stroke length and applied pressure along push- pull travel path. If the polish( grind) by a machine, theoretically, the optimal process to form a sphere surface is 43.3% stroke TOT(see below). The range is around 1/3 stroke + 15% to 25% overshoot ( meaning 38.3% to 45% stroke) So actually, the past literature of ATM books suggest 1/3 stroke to achieve a sphere, is assumed that the person will overshoot the stroke length. But if overshoot is too much a hole will start to form in the centre. Reference: Computer control dwell time algorithm RFITA