First, this will probably work just fine, even with assumptions. People like us who are not true experts on things like airflow usually overengineer things, so I wouldn't worry....

But where's that German guy with the Indian sounding name (Gurhard?) when you need him? I ask because THIS is a situation where his information might get us an exact answer, if we had all the unknowns to plug into his formulas. And that's a big IF.

One fact is that every opening does "create... additional static pressure.*" A HUGE opening would create very little static pressure, perhaps negligible; a small opening will create more.

Another note from an article on this is "Fans of the same diameter are not all created equal. Airflow capacity at different static pressures varies depending upon fan make and model."



*"static pressure" is an odd term as it refers to the push that a fan gives to air as well as the resistance to air flow given by a constriction. It's as though voltage and resistance were replaced by a single word.