The off-road rock-crawler truck (Jeep etc) has terminology for this. At the bottom of the ramp where it transitions from level to uphill/downhill (depending which way the vehicle is moving), it is the "approach angle" at the front of the vehicle and "departure angle" at the rear of the vehicle. At the top of the ramp where it transitions from sloped to level, it is the "break-over angle".

A generously rounded corner, instead of a sharp one, could reduce the required ground clearance / break-over angle, while maintaining generally similar ramp lengths and slopes.

It's unclear from the scenario presented, what you are asking us to help you with. Explain to the rest of us why you can't make the ramp longer and less sloped. Explain to the rest of us why you have to accommodate a vehicle with extraordinarily low ground clearance whose owner knows it's going to scrape the ground everywhere. Is there a specification for what approach/break-over/departure angle that you have to accommodate?

Google "ground planes" or the aforementioned approach, departure, and breakover angles, then make yourself a simple spreadsheet. Its all trig.

Why can't you provide vertical curves instead of grade brakes?

I've attached a figure from the NY State Department of Transportation's Highway Design Manual that may help. Based on the use of metric units and the silhouette of the vehicle, I think it's based on an early 1990s Chevrolet Caprice station (estate) wagon. You could measure the cars in question and develop a custom template to use with your drawing.

My glass has a v/c ratio of 0.5

Maybe the tyranny of Murphy is the penalty for hubris. - http://xkcd.com/319/

Good idea do any figures diagrams or simulations with the bottomed out heights of the suspenson.