[M_a_t_t]

So I was considering doing a partial Kammback on my van (95 astro van) and I was thinking and I realized that most, if not all, I could find as examples are on cars where the end connects to the bottom of the hatch glass area.
If I follow the template would it still get me noticable fuel economy?
Would the flat back not matter at all?
MetroMpg:

AndrewJ:

What I was thinking: I didn't try to get it accurate with my painting
(see thumbnail)
My question is how much would it help for a vehicle with a flat backend?