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In Souter's Book " Star Testing Astronomical Telescopes", p82 he quotes Welford who recommended 20 times the focal length as a minimum distance to the pinhole.
He also gives this formulae:
N(ft) = 28 *(D/F)(D/F) (D in inches) where D is the aperture and F is the focal ratio.
To achieve a good Airy Disk, the size of the pinhole when viewed by the telescope should be close to the Airy limit, for that scope.
ie for a 200mm" f5 he gives a pinhole of 0.14mm diameter.
I've recently converted my old Foucault tester and fitted a 50micron fibre; this gives a very good starlike image. I didn't use the LED solution as I can use the fibre with various lamps ( flouro, incandecent and Neon) to help in testing spectroscopes.
Hope this helps.
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