Before you get too extreme trying to follow "the template" for your top, let me give you this food for thought. I drive a 2007 Dodge (Mercedes) Sprinter high top cargo van. These vans are very fuel efficient for being a brick. I believe part of that has to do with what they do on the very back of the vans.
Here is a picture that somewhat shows what I'm referring to:
The part to look at is where the tail light housing is, and the body line that extends up the side from the front of that housing. From that body line back is only 6-7 inches. However, it is a smooth curve that ends at about 15 degrees inward from the side (from what I can measure with a straight edge and a protractor), and it is a "sharp" edge, providing a clean separation for the air. This same curve happens on the top as well, which you can see in the picture. I believe this setup, I'll call it a micro-boat tail, most likely tested in a wind tunnel extensively by Mercedes, must be just enough to suck that air inward and reduce the drag on the back of the van.
So I guess what I'm saying is, if you want to maximize your interior headroom from front to back, you might not have to sacrifice as much as you think. Obviously, the farther you go down in the back with the template will reduce your drag that much more. I just figured I'd present you with another option. Hope it makes sense and helps.
