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Alex, I think you just divide the focal length of the EP by the f/ratio of the scope.
A 31T5 in my f/5 scope produces an exit pupil of 6.2mm which is at the limits of what I should be using - much better with a Paracorr, which reduces it to about 5.3mm.
The example you have given is unusual in that you have referred to 65mm EP. I have never come across one of them!
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