being able to round the corners at each slit end, so that the parasitic spikes will disappear;
being able to choose the width of the vertical bar (the one that separates the half circle slits to the quarter inclined slits)
the F/S factor can be easily calculated. Its value for +/-20° inclined slits is 27.5*N where N is the telescope aperture. I explain how it is calculated here: https://sahavre.fr/wp/masque-de-bahtinov