Did you figure this out? I just did this on Saturday on my "quick-n-dirty" 8-string conversion. Granted, I wasn't very scientific about it, but I spaced the primary strings first, leaving enough room for the E octave string, then I fit the octave strings into the space left over. Make sense? Basically, think of it as slotting a regular four-string nut in an offset position, and the octave strings the same way on the other side. Graphics would help, I'm sure, but I don't feel inclined to break out MS Paint right now...