Here's a picture of three aerodynamic vehicles in profile:
And Les Baru:
Note how the Insight, Prius, and CRX have roofs with gradual transitions. I wonder whether flow remains attached across the corner at the back of Lesbaru, where it transitions from nearly -0° to -15°.
You need to determine where or whether you're getting a turbulent wake. Tuft testing is best, but you can also study the behaviour of road spray. When I'm traveling at highway speeds, rain doesn't land on my upper hatch glass, but the back of my car gets covered in road spray, just like the back of a Dodge Caravan. The outer surfaces of Chang's homemade roof extensions stayed clean while the inner surfaces got dirty.
So if you start with a clean boat tail and drive on wet roads, the portions that are dirty when you finish are probably turbulent.