I would like to state that I am not criticizing anybody here, just a general viewpoint.
Not wanting to labour the point or anything 😬, but of what practical value is knowing your Cd? It's only use, at our level, is for comparison purposes, or perhaps to impress your friends by having a significantly lower Cd than their's 😬
Consider a standard 7, it has a Cd and a frontal area. If you replace the standard windscreen with an aeroscreen you will reduce Cd and frontal area. If, for instance, you now fair in the cockpit, Cd will decrease, but not frontal area. There's a WHOLE lot more to the Total Drag of a 7 than those two characteristics.
The practical suggestion of establishing a base value of rate of slowing down and then making a physical change to the car and carrying-out the same slowing down test will tell you if the change(s) you made are worthwhile.
Anything other than a complete and radical re-design of the 7 will achieve about 10% of bu**er-all difference - aerodynamic wishbones - "give me a break" ❗
And, IMHO, sticking bits of stiff cardboard onto a 7 in a wind tunnel for a couple of hours probably generated only amusement among the operators 😬