Here's the first gen Insight (Cd .25) and the new E-Class:
I scaled the Insight to the same height as the E-Class, and tried to line them up so the peak of each car's roof are at the same point.
Note the backlight angle on the hatchback essentially points at the end of the decklid of the Merc, an example of the "effective" angle of the back of a 3-box car being the line between the trailing edge of the roof and the end of the decklid.