I think there is a simple formula for this, that i have long forgotten. But I'm
sure someone here will know. Like I said I forgot the formula but I think I
remember it went something like....
focal ratio of scope <divided by> focal length of eyepiece
which works for your 42mm and 8" scope above.
Would 8.4mm be too much? well that probably depends on how large your
pupil can dialate, if it could even dialate to that size.
excuse the typo's : )
regards,CS