Recalculate zone floor area (ZFA) = (WA xVLT)/0.150. So if, for example, you have a window area of 120 ft2 and a VLT of 0.4, ZFA = (WA xVLT)/0.150 = (120 x 0.4)/0.150 = 320. There is one caveat and that is that your ZFA must be equal to or greater than your zone width. Otherwise you must enter 0 for this ZFA.