It's cloudy, so what to do? Previously, I've sent an email to Sibert Optics to ask for the optical path length of my Black Night, however, I didn't get any response, and I guess my email was probably trapped by the spam filter. So, why not measure it myself?
I pulled out my Borg 45ED II:-
I found that I can reach focus without a diagonal... so, it's time to remove the binoviewer and to use the eyepiece alone:-
I found that I've to use two extension tubes with a total length of 11cm and I also have to extend the drawtube on my Borg by around 1cm more. Therefore, the optical path length of the Black Night binoviewer should be around 12cm...
So, if I want to use the binoviewer without corrector with my Orion 100ED, I should then cut it by around 12cm.
After this experiment, I pulled out my Canon 10x30 IS for a quick comparison. The binoviewer Borg yields around 16x (with two 20mm Tele Vue Plossls). The view from the Canon is much brighter, and the 3-dimensional feeling is also stronger. Anyway, there is really not much a reason to use a binoviewer with a 45mm scope.
Harry replied and it's 12cm officially.