I am not familiar with Maxwell's numbers, but I would bet that the angle stated has to be relative to the airflow. For example, assuming the air "hugs" the downward slope of the rear of a coupe, then it would probably be 11 degrees off the downward slope. If the airfoil extends further away from the body, it might require less if the angle of the airflow is closer to parallel to the ground, but it might require more if it ends up in a turbulent area of airflow. Other factors to consider include the air fold shape, flat plate vs. cambered (thickness) and surface area.

Start at 11 degrees from the downward slope of the body. Then try it and adjust. It will require some time, unless you have access to a wind tunnel.