[MetroMPG]

Your protractor doesn't lie.

The reason I can get away with 15 degrees is because of the gentle curvature at the back of the roof & upper surface of the hatchback leading up to that final angle. Also, the "shoulder" of my Kammback has a generous radius, permitting flow to transition from the sides without tripping.

If you use a "sudden" 15 degree angle/transition from your cab roof to the aero shell, you could end up with separated flow and dramatically reduced effectiveness of your hard work.