[aerohead]

The SAE has recommendations for approach,breakover,and ramp clearances so none of a cars naughty bits are torn off,scraping on a driveway ramp,railroad crossing,etc..
For the front of a car,a 16-degree approach angle is recommended,as measured from where you can just see daylight between the front tire tread and road surface and any part of the car ahead of that point.
If the airdam extends too much your drag will go back up as you add frontal area.
If you're willing to modify your driving technique you can suffer through with a lower dam,but you'll have to approach some road hazards obliquely so as not to encounter a ground strike.