Interesting, I know the formula for working it out, I know the optumim is
around 2mm, and when the exit pupil is larger than the iris of your pupil
you get an effect similar to sky glow. Correct me if I'm wrong, others
will be better at explaining it than me. But I think the effect could be
eye placement maybe more critical. But I'm not really sure.

John Bambury would be the person to ask. I think he was the one
who showed me the formula, and possibly a few other things in regards
to exit pupil and the effects. But unfortunately I forgotten...

Sorry I cant be of more help.

regards,CS