Measuring the angle with GIMP, it's about a 16 degree angle from the roof to the tip of the spoiler. With no spoiler, it's roughly 19 degrees to the trunk edge. The window is about 35 degrees.
To mount a kammback at a 13 degree average, I'd have to extend the spoiler another 3 inches.
I've driven 130 miles so far on this tank and I'm about a gallon or so above half, so we'll see what the tank average looks like around the 7th.