Not sure what you mean by terminal junctions, but here's what I did:
Drew out the catacombs, calculated the total volume of all the passageways (all vertical then all horizontal), the subtracted the extra volume from crossing over the intersections twice in the way I calculated the passages.
I double- and triple-checked my calculations and the method used, but haven't confirmed my answer with anyone else so I don't know how it compares.
EDIT: I understand "cul-de-sac" now
From the way I looked at it, there are no cul-de-sacs, the last passageways simply end 25 meters from the center of the previous intersection.