Reynald's number
As someone once said - - experience helps .
On race cars - the use of an "airfoil" is really a setup where cut and try works
As for aspect ratio on a car -- the aspect ratio on a car wing is very low - and must be so ---unless you consider a 3-1 aspect ratio to be high--
Are formulas of ANY value here?
Of course -of course--but
One of the best aerodynamic guys in the car business was a guy named Kamm -- who came up with a very interesting bit of logic.
"If you can't make the airfoil work for you - get rid of it"
(Ever wonder why all of those high speed road cars had that squared off rear end?)