My understanding of exit pupil is aperture/power. So with my 20mm EP and a 400mm focal length of the PST gives a power of 20X. Aperture is 40mm divided by 20 = 2 ie exit pupil of 2mm. This is fine by my ancient eyes!
Which is why I cannot see how the Barlow makes a difference since, by its addition the power should effectively be doubled. Therefore 40/40 = 1. Thus an exit pupil of 1mm - even better! Your formula is a different matter and may be the clue to what is bothering me. Yet it makes no difference on the 200mm LX90! I think I will just go away and meditate for a couple of hours to get rid of the headache I have thinking about it!
Cheers!
Bert