[elhigh]

Nice. Tape stripes FTW!

Take a good, beltline-level shot of your truck, from right smack at the trailing edge of the door, from a modest distance, 50 feet or so. You want some distance to eliminate parallax error (is that the right term?). Then you can blow up that image and drop the aero template on top of it (find it in the sticky in the Aerodynamics section), scale the template so it's the same height as the image of your truck.

Then you line up the highest point of the template with the farthest-aft high point of your truck's cab. The template line from that point should give you a good line to follow with your aero cap, if you really want to max out its performance.

To get the fullest benefit of an aero cap, you want to take it just as far aft as you can. One guy even built an extension box on his truck and the cap then displaced his tailgate - I can't remember his name right now. It looked pretty sharp.

But that's the expensive, difficult way. If you're a talented fabricator in your medium of choice, it's not as difficult or as expensive as it is for the rest of us mortals. But you can get a lot of the results with a well done straight cap. Just keep its straight line as close to filling out the overall line of the template as possible and you should be good.