I did some template overlays to study the shapes, the estate/wagon shape is close to but slightly higher than the Part-C template. Meaning slightly more adhesion at the roof than we need. It also results in a higher rear wake area resulting in the marked higher Cd number.
Industrial Design - Transportation Photos by kach22i | Photobucket
Scaling the template as silly and as improbable to succeed as it at first may appear, does a good job at predicting attachment (see Audi TT wind tunnel images posted earlier).
Industrial Design - Transportation Photos by kach22i | Photobucket
From the SAE paper we see the Fastback has the best numbers. My take on this is that it uses the best combination of least amount of rear area (wake), and largest template shape.
The template path to a Kammback works (history supports this), but if you can maintain attachment plus get the smallest rear area possible to reduce your Cd, then you are on the right track.
This is all conjecture as I've done no scientific testing, but it's conjecture based on old school observation. Learning to observe (without bias) is the basis for science and art alike.
This same sort of methodology started the Renaissance Era and gave us guys like Leonardo da Vinci. If any of us can be a fraction of as a good observer as he, then we are having a pretty good day.