If you are using the speed of the car in your calculations, you will probably get more downforce than you are calculating. The downforce should be calculated based on the local velocity of the air in front of the wing. As the air accelerates over the car, the air velocity near the wing should be higher than the speed of the car. However, depending on how far away the wing sits from the body, and the turbulence of the air near the rear of the car, the local air velocity could be much less than the speed of the car. Your calculations should give you a good starting point, but as Top_Down_SRT said, it will also require some practice to fine tune it. Also, the airflow over one wing section will affect the other. Make sure you are using equations for a wing with a flap, and not looking at each wing separately.