I been thinking about this some more, I wonder if ending the Kamm before the rear end of the car isn't such a good plan, here's why.
If you create lower pressure over the area above the trunk, the higher pressure air along the sides is going to want to fill that void more so than normal, so it will be pulled upwards, and by the time it reaches the downwash from the roof, it will be pushed downwards. This may be creating larger vortices trailing off the rear areas above the tail lamps which would be a real drag.
Like Aerohed says, by making it longer and making the sides taper in less as well as keeping the sides in place further back, I think this would make the Kamm effective. Also, it seems anything you do to radius the surface will help the shape create a lower Cd too.