To clear up some of the above,
Draw a line from where the tires hit the ground at 16deg. Don't hang nuthing there.
Normal thinking is stop the air dam level with the lowest hanging thing under there. Probably the bottom of your axle.
Self drilling sheetrock or lath screws go right through heavy truck bumper steel too !
I'm working on a similar dam here :
http://ecomodder.com/forum/showthrea...dam-18663.html