I'll have to experiment with adding a flat "step" to it then.
Update: This morning, the car was coated with dew, and on the usual commute the dew showed little signs of airflow over the rear window and trunklid. I'll probably do some tuft testing to better see what's going on and if the air is getting trapped and making that "virtual" angle.
I'm thinking of getting some kind of stick or something with tufts up in the airflow and see how the air is flowing above the surface, then measure the angle of airflow from there.