I hate to say it - but... YMMV.
Having just read most of the Simon McBeath "Competitions Car Aerodynamics", what has been driven home to me most strongly is that there is no rule of thumb applicable for aerodynamics. You cannot apply what had been done with a truck with a dirty underside (frame, axles etc.) and substantial ride height and apply it to a monocoque body just off the ground. Some of the airflow stuff shown by CFD in the McBeath book just boggles the mind...
I'm thinking about setting up for CFD at the moment (Linux, Gerris solver etc. etc.). I'll only try 2D modelling due to the time required for a 3D matrix, but I think it's going to be worth the effort.