I agree with cfg, angle schmangle, you only need to extend the roofline back following the ideal aero template to achieve the best result.
I'm pretty certain you have attached flow on the back of your windshield now all the way to the spoiler. The Kamm would only add a small amount of efficiency by making your car closer to the ideal aero shape, essentially eliminating a lift creating area on your back window which is also creating drag.
The kamm should only need to come back a foot or so to achieve the desired effect. If you go back further, it will be more difficult to build, become counter productive due to more skin drag being created, and I think it would be ugly.
Hope this helps.