Gah - a solution with more questions. – EntropicLqd
This window holds the options for rebuilding a map
The process itself is described in Build
The Stats Tab
Max Depth, Avg depth: no idea
A map made ofa single cube gives these two as 5 and 6. How the Average can be higher than the maximum of somewhat baffling.
Again, the single cube map gives: 6(0). Adding a light makes this 7(1). Adding a ton of cubes seems to make no difference. Weird.