Request for Info re CI and 60% Gross Floor Area Calculation

I am not too clear on the definition of Gross Floor Area and how is the calculation carried out with respect to the CI requirement of alteration to at least 60% of the gross floor area. I thought I remember reading something like it includes floor, ceiling, walls, etc. but I can't seem to find that reference again. So for example, if the tenant area is an entire two floors with mainly an open office arrangement (moveable cubicle walls/offices), would floor represent approximately 40%, ceiling 40% and walls 20%? And if the raised flooring system will be reused but all carpeting will be replaced, how does that count on the flooring percentage?