Design the Kammback just like a boat tail, but cut it off at the back of the trunk. You should use all the same angles as a full boat tail would. If you're not sure just do a cardboard mockup with 10-15 degrees on all sides and ABA coastdown test it. (and tuft test too)
If you can get a straight and level photo of the car (or better, a blueprint-I had no luck) then just input it
here to overlay the template. That will give you the ideal profile.