A Model A is not an aerodymic complexity. In fact, the windshield represents pretty much a right angle obstruction to airflow. Maximum airflow through the cabin is obtained with the windshield wide open with a rear window (if available) also open to allow flow. Having said that, that’s also the noisiest in the cabin. Take your pick what you need.
Also keep in mind that with outside air above about 85 F you might be actually gaining heat load with more flow. It’s a balance between evaporative loss and conductive gain.
If I want more flow I generally open to the notches, open the rear quarter windows a bit (ah, the joys of having a Town Sedan!), and call it good. Not terribly scientific, I know.