I don't have a formula either, and I don't think one would even be useful given how different various cowl setups are. Little areas of stagnant air negate a lot of the benefits of the flow that you do have for example. From what I've seen though, it's more about directing the air through the cooling fins than it is about the total amount of airflow. Baffles that don't allow the air to go around the engine instead of through it do restrict airflow but improve cooling. So if you want to make it scientific, it's actually about the amount of air flowing through the cooling fins, not how much air is making its way through your cowling.