1/3 is rise over run, same as tangent of the angle...so arctan .33333, or 18.43 degrees (.322 radians). Roof pitches are done the same way. I prefer degrees over percent or pitch (ratio); just my personal visualization bias.
I'm sure the cab raises the COG a bit, but I don't think it raises it as much as it LOOKS like it does. On a larger tractor, the cab adds considerably less than 10% to the weight and, as a previous poster pointed out, a considerable portion of the added weight is located down in the engine compartment. Except for the glass, most of the high stuff is plastic, electronics and air space.