i would guess you take your ride height, and the distance from where the front of the tire makes contact with the road to the nose of your car, form a triangle, (actually subtract an inch for tolerance from the ride height (or more) ), form a triangle, and figure that the approach angle you want will be the....arctan^-1(RH/ND)
with arctan^-1 being inverse tangent
with RH being ride height (minus 1)
with ND being distance from front of contact patch to front of nose
also, probably make sure you are in degrees mode on your calc.
I AM GUESSING THIS WOULD WORK