Post #5 makes it fairly clear that you have the numbers correct. A 1200mm focal length gives you a total width S of 6-8mm, so the slits need to be 3-4mm and spaced by the same amount.

I'm not clear at this point whether this is a major improvment over an ordinary Hartman mask which has 6 diffraction spikes. The triangular version gives 18 diffraction spikes, which is really unusable, but if you paint three toothpicks black and attach them to the holes of a 3-circle Hartman mask, you get much the same effect I'm seeing in the animations.

The main difference would seem to be that there are a lot more slits and the specifics of the slit spacing. Perhaps this yields brighter diffraction spikes?

- Gus