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There is a difference between detecting astigmatism and seeing astigmatism in the in-focus image.
For example, I can detect astigmatism in the eyepiece down to 5mm (0.9mm exit pupil) by defocusing a star on both sides of focus, looking for ovality in the star, and evaluating for a 90° change in orientation from one side of focus to another.
But I do not really see astigmatism in a focused star image in a 10mm eyepiece (1.7mm exit pupil), but I do in a 12.5mm eyepiece (2.2mm exit pupil).
I have 1.0 diopter of astigmatism, so their chart predicts I will see astigmatism above a 11.5mm focal length (2mm exit pupil).
Your formula suggests I would see it at an exit pupil of 1.5mm and larger.
I can see it in an out of focus star image at a smaller exit pupil than that, but don't see it in an in-focus star image at that exit pupil.
I recognize that if it is even detectable in an out of focus star image on either side of focus, it is affecting the in-focus star image in a deleterious manner, but if you see the star as a point with a single diffraction ring around it, how badly could it be damaging the star images?
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