Daox is right: to use the template properly, the bottom of the teardrop should be on the ground plane (with the vehicle's tires) and the highest point of the vehicle's roof should line up with the highest point in the tear drop shape.
That would show ideal curvature/taper, or close to it.
Having said that, Phil's first T-100 aeroshell was angular/flat planed, and it worked:
But his curent one is literally made out of a boat hull (compound curves), and may be better.
Also, you could safely mimic Bondo's F-150 aeroshell - it also works.
Commercially produced aerodynamic pickup bed cap
Just eyeballing your image and comparing it to Phils & Bondo's , I would guess that it would work fine (though I would radius the shoulders a bit). The "ideal" image would have slightly less of a slope.