How is the edge margin (distance from the groove to the end of the shaft) calculated for a spiral ring?

The edge margin, or 'groove location' $Y$, must be sufficient to prevent the shaft material from shearing off under load. The minimum distance $Y$ is typically calculated as $Y = \frac{K P}{D \pi S_y}$, where $P$ is the thrust load and $S_y$ is the yield strength of the shaft material. For most applications, a rule of thumb is $Y \ge 3G$ (three times the groove depth). If the edge margin is too small, the material between the groove and the end of the shaft will 'blow out' in a shear-tear failure mode, even if the ring itself is capable of carrying the load.