I'm not quite sure where that came from. The lift height shouldn't be in any way reliant upon the cushion area, unless the attendant weight gain from the extra material overcomes the lift pressure. Adding weight, such as passengers, will initially raise the pressure, but the excess will almost immediate bleed out. That is dependent upon the skirt design, though. Something rigid, such as a single long strip of rubber, will hold more pressure than the preferred multiple 'mini-skirt' type. Regardless, the pressure will always equalize automatically. Too much will leak out; not enough just won't lift. The lift height is typically measured from the lowest 'solid' part of the machine, since the skirt is always in contact with the ground. That solid part can be the bottom of the frame, or the upper part of the skirt that isn't flexible enough to move easily. The measurement is essentially an indicator of how large an object the craft can pass over without hanging up, like the ground clearance of a car.
