I could really care less about the shape and or relative sizes of the middle and top. I'm a negative utilitarian, so my only concern, really, is with the bottom. First consideration is minimizing the number of people "below poverty line" (although I think the line between survivability and not is a little more complex an issue) with high priority. Second consideration, is raising the lowest point in the configuration. Assuming levitation or powered/unpowered flight is somehow not permissable/possible or is outside the parameters of the exercise, I suppose Brin's concentration of the weight of society on a very small base area (a single point) is the optimal satisfaction of the first consideration. Also, if the lowest organic (i.e. human) portion of the structure is above ground, supported by the inorganic portion (say, automation), that is also acceptable, as long as the automated industrial substructure can support the entire social structure sustainably. If not, my next suspicion is that there are too many people. But don't blame me, I'm child-free.