Huh? If its engineered and not 'from the hip with a huge fan' its right, as opposed to the guess work and after thought. Turns, leaks, pressure drop over a filter, etc are common factors for an engineered design.you could in theory engineer a system the way your questions suggest. or, you could buy an oversized fan with a variable speed controller and adjust the fan to meet your needs.
the theoretical approach has so many ways to go wrong (not accounting for turns in the ducting, leaks, equipment like carbon filters etc) that you're only getting a rough estimate anyway