Overlaying the AeroHead Aero Template on the Insight, and then importing into a small CAD program, the following angles of curvature were measured...
Since Robert did some tuft testing, and the yarn stuck cleanly to the rear glass, that means that our "angle-of-departure" can increase to match the Insight rear hatch angle and still be called good, compared to the rate of change for the Aero Template.
After measuring the rear deck angle on the Insight, I was somewhat surprised to find out that the angle does not approach 18° as first thought. It does instead, match the angles of the Aero Template rather well.
When starting to layout the angles-of-departure for the tail extension, I will make a master template from foam board and use this to layout angles for the top and sides of the extension, and the underside will certainly be stepped to achieve at least 6.5° as Metro has previously mentioned.
Jim.