Lift/downforce physics info??

[j.fro]

I'm doing a little research on the downforce produced by different Porsche rear spoilers. I know the lift equation: Lift=(lift coefficient)X[(air density X velocity squared) /2] X (wing area); however I cannot find a formula for the lift coefficient that doesn't simply change this formula around, requiring you to know the lift as one of the factors. Anybody have an easy lift coefficient formula?? Or, just to make things really simple, does anyone have downforce numbers for Porsche spoilers? In particular, I'm looking for the Boxster spoiler, the 930 whale tail, the 996 GT3 wing and the 997 GT3 RS wing.
Cheers!