good points on the complexity of 3D shapes and flat side surfaces. The on course airflow may not agree precisely with the paper area calculations for a particular vehicle and all of our cars are going to have some divergence from our simple calculated results. The point of the exercise is to get a base line idea of the high speed stability of our cars. Wind tunnel time will validate (or not) our stability assumptions but not all of us have access to these great devices. Grease dots are a time honored land speed method of seeing where the air goes around a vehicle at speed, taped yarns and an on board camera are the information age replacement for grease dots, suspension data acquisition is great info as well; stiff wire, zip ties and tape can provide simple data in place of sensors. Weigh the car, figure out the area center of the side of the vehicle, do the cut out test then make some assumptions and next summer spend a few work up or test runs proving or disproving the stability assumptions. The 411 streamliner was designed by a young mechanical engineer using paper and pencil proving to me that good results are possible applying the basic principles of mass in motion traveling through the air.