It really is not about air circulation, it is about air pressure.
Drive a car down the road really fast with only the drivers side window down. The air in the car will be overpressure dut to a large volume of air being driven into the car.
However, circulation is for shit because you have not given the air anywhere to go.
Now, open the passenger window and we really have ventilation happening. The car is still overpressured but because we have provided an escape we now have easy airflow.
Negative airflow is harder to demonstrate but if you go to a very large building such as a shopping center and open the door the air will either rush in or out. If the air rushes in it means the building is under negative pressure and that the outflows (ventilation) is exceeding the inputs.
That is also a demonstration of scale... You can have positive and/or negative pressure in something smaller than a soda bottle or larger than an enclosed stadium.
Anyway, there is no easy way to measure your actual airflow due to impedence of ductwork, different equipment, etc...
The point is to try to generally have more air exhaust that you have coming in. If you have a 4" inline duct a 6" exhaust duct with a fan would be nice... If you have a 8" inline duct maybe a 10" exhaust duct would be good, etc...
Otherwise depending on how tight your setup is the air will leak out from every little crack and that could allow the smell to escape too. It also shows like the car with only one window open how not allowing for exhaust can mess up both sides of the ventilation system.
Hope that helps.