Well - - - - -
The Kamm would start at the back of the cab roof (more or less) and go down and back to the top of the tailgate. Ideally, it would be gradual curve, as opposed to a straight angle back and downwards.
I know there are some threads in the Aerodynamics section here where people built what we're talking about, for trucks.
Done right, it would help the truck body take on the shape of the ideal "template" curve. See this thread for the template:
http://ecomodder.com/forum/showthrea...rt-c-9287.html
Having learned a bit about using the template, I can say that one important point is finding the right height that the new bed cover needs to arrive at, at the rear of the truck. Depending on the existing truck shape + the length of the bed, it might need to end higher than the top of the tailgate - or not. If it slopes down at too steep an angle, the air won't flow along the panel, instead it will break away with resulting turbulence.
You want to coax the air to follow your new panel downwards, and then depart from it with the smallest cross section of wake you can get. Basjoos did it on the aerocivic by building an extended tail that tapers down to a panel just big enough to hold the license plate and required lights. On a truck you can hopefully taper it down to the size of the tailgate - but only if the bed is long enough to build that taper with a shallow angle or curved line.