PT Aggression Factors.

[tvta]

I searched all of 2+2 for "aggression factor" and nothing came up. Could someone tell me how this number is calculated?? I remember reading something along the lines of [(bets+raises)/calls], but I am having a hard time applying these numbers when in game. I simply go by the default colors by PAHUD to let me know what I am up against. The only time I find myself applying these numbers is if I am unsure of my hand and a <1.5 agro factor pushes hard, easy fold. But I am unsure as to how to take advantage of higher aggression factors since 3->inf are listed as red. For example, I fail to distinguish 3.5 from 9.