Gosh, I've learned a bunch about optics in the last few minutes of reading this thread!
Just use the pythagurous theoretical, Naught plus Naught equals Naught, carry your Naught and you have Naught.
I like Ingwe's example the best, just like flowing water a 30mm tube lets in more light than a 25 mm tube. Then based on the widely referenced and annotated Swedish study of looking a partially clad women in the total darkness I find I need a 72 mm objective with a 34 mm tube which lets many out of the running.
