The difference in focuser position between these three pictures is very small, a few tenth of a mm. The method is incredibly sensitive. And much faster than FHWM, or anything else I know (yeah, autofocusing also). It also works for visual! You don’t need to guess if you focused well (when air is unstable) just put the mask and everything is absolutely clear. You have to try to believe. It works for different scope sizes and focal ratios. The general rule to holes’ width is f/s=150-200, where f – is focal length, s is hole step size. So for telescope with 1500mm focal length the step size is 1500/150=10mm (5mm hole, 5mm for uncut space). If holes become too small the formula becomes 3f/s=150-200. eg for 800mm scope you need to cut 2400/s=200 s=2400/200=12mm (6mm cut +6mm uncut).
The angled cuts are placed at 40 degrees to each other.
I cut mine at laser engravement services for $30.