[skyking]

First pic shows the basic line moved aft per Aerohead's suggestion that I was forward of separation point. Of course the final product would have a curve there.
Next few the basic cardboard is on, shows that relationship between kamm and sunroof, etc.
Back is too square for a curvaliscious car like the beetle.
Added a curved tail, with view from behind too.
Final has the Colani curve tacked on. I think it will move aft and there will be more kamm there at the notch.
The bottom end can go aft farther without impeding views. I could also add a shelf wing down there out of sight if I can make it work.
None of it was visible in the mirror, only when I turned to look back and to the right, and it could go a little more aft as you move up the trailing edge.