Just [load supported]^2/[mass of tower]. Personally, I think the scoring system is a little weird, but I do like that they're trying to put a little more emphasis on holding more weight.
HSciO 2014 — Astro, Forensics, Mission, TPS
"A common way to compute density is, of course, to take the ratio of an object's mass to its volume. But other types of densities exist, such as the resistance of somebody's brain to the imparting of common sense..." ~Neil deGrasse Tyson