Ah! Here it is:
http://ecomodder.com/forum/showthrea...rt-c-9287.html
Length-to-height ratio of 2.5 for the back end. That calculates to about 24 degrees. (update: wrong - see below)
If you measure the angle from the rooftop to the trunk trailing edge of your car, it's likely a little steeper than that. You want it flatter, so make a lip that extends out the back a short distance.
Edit: Wrong information! Sorry!
That's 24 degrees between the top angle and bottom angle, but only 12 from horizontal to either one. 12 degrees is what you want at the back of your car.