|It would appear that you are discussing commercial buildings by your simplistic understanding of exhaust and supply air volumes. That is only one part of the pressure differntial equation. The other factors include wind pressures and stack effect pressures.
More correctly you need to be using a manometer that measures pressure differntials between the interior and exterior to acurately determine both the amount of pressure and if it is positive or negative to the building interior. And since these are variable you should have a good idea of the variability during the inspection. You can use an anemometer to determine the specific air flow through any one area of the building enclosure in combination with the manometer readings.