You're on the right track to passive air intakes but basically instead of worrying about how to become a rocket scientist and calculating your pressure, just take a single 2 ply piece of toilet paper and place it over your passive air intake, if it holds itself there just enough to where it doesnt fall, you're doing good/your intake and exhaust are working near perfectly. If it falls off, your intake sucks/it's too big/you need a bigger fan. If it gets sucked in, your intake isn't big enough/your fan is too powerful for your intake size. A toilet paper piece might not be large enough if your passive intake is huge, but you get the concept. Apply however necessary.