Just a thought but perhaps it would be worth making up a test panel and trying it on a bracket outside your car window. The pivot bearings could be mounted on ply sub ribs so the pivot can be moved fore and aft from the 20 to 30 % points in a search for the aerodynamic center. The "strain guage" would be your hands on the angle lever. Move the pivot point until you notice it self stabilizes but has a fairly consistenly low effort to change the angle of attack over a 20 to 30 degree range.

I'm thinking of a 2x4 L that locks to the passenger door to hold the test panel out about 2 or 3 feet from the car, a shaft of metal tubing with some plastic bearing type pillow blocks on the arm of the L and a cheapo 4 rib wing panel with Monokote. The sub ribs would have slotted holes to let the pivot axle tube move back and forth and then tighten in place. A quiet road and a 1/2 dozen runs would settle where you want the pivot and give you an idea of the servo loads.