[brucepick]

Thanks for this. Good news.
I did the calcs for this, for my car and I like the results.

I get an angle of 3.69º for the diffuser, using existing available attachment surfaces. That's assuming the panel prior to the diffuser is horizontal, which I believe it is. Pretty close, anyway. The rear bumper lip is 2" higher than the existing support I'll use to support the diffuser's leading edge (which will also support the prior panel's trailing edge). The diffuser is 31" long at the center, so that's an angle of 3.69º per a handy calculater on the web. Allowing for the bumper's curve, if the diffuser length is only 25" at the sides, the angle is still 4.57º. According to the chart you posted, either case gives a reduction in Cd. Thanks!