Is there a standard formula or rule of thumb for how much airflow a whirlybird/turbine roof vent will achieve given a diameter and a wind speed?

I've been looking for effectiveness measures for whirlybird vents for a while and I can't find anything in building codes, and only one empirical claim from askthebuilder.com for a relationship between diameter, windspeed and airflow. I know there are general accepted rules like 1 square foot of ventilation per 300 square feet of roof space, but I'm really looking for a formula for airflow. Am I oversimplifying the problem? Thanks!