Along the same idea of the excellent ancestor : one screw hold the endstop, the second hold it but also tighten the printed part. The difference is this one place the endstop closer to the rod.(fyi : there is 17mm between the M3 screws center-to-center, in case it can be used for microswitches)http://reprap.org/wiki/Hall-%CE%98Add 2 M3x20 screws and 2 m3 nut between the pcb and the printed part, then put on your M8 rod for an accurate homing (e.g for the z-axis) :)