I've read that fortifications were a mathematical formula in the legions that X number of men = Y size of fortification and scaled it up, how accurate is that? And how long did it take for them to build a fortification?
It wasn't exactly that x men = y size of fort; the legion was a standard size, and thus there was a standard camp, with adjustments when needed.
A good source here is Polybius, who gave the process (as it was in his day, before the Marian reforms; I don't know whether Marius changed encamping methods as well as armaments and tactics) in great detail; it's available here (start reading at para 27): http://penelope.uchicago.edu/Thayer/E/Roman/Texts/Polybius/6*.html