Phil, thanks.
Cyclist916, here's my understanding. It's not
just angle. Smooth flow is key. For most trucks I suspect a simple wedge from cab to tailgate is better than open box by stuffing the wake. It still leaves room for improvement via sudden change of slope. That discontinuity in shape detaches flow. Arch the lid for clean transition to keep flow attached. Keep the front tangent to the cab. On a long bed the top might be able to arch all the way down to the tailgate without detachment. On a short bed the top may have to stay somewhat higher than the tailgate. It really depends on the truck. Other factors include shape of the cab, hood and grill, plus how well the aerocap fits the cab. The cleaner the transition, the better. Tuft testing of a cheap prototype should reveal max curvature for your truck. Also round edges for smooth transition between top and sides to minimize vortices. No one design (shape, curvature, angles) can be "best" for all conditions (wind speed, direction) so generous trailing radii make it robust, forgiving, tolerant. This is my understanding. It may not be perfect. Hopefully others with more experience can correct these guidelines. Good luck, and please keep us posted on your project.
Cheers
KB