I can't prove it,but my opinion is that,unless you can also taper the bed (which would require a custom bed),just keep the sides of the cap all the way to the bed rails,rolling the sides, such that as you reach the tailgate/bumper area,that upper edge will be like an ellipse coming off the rail top over onto the top of the cap.
If you decide later to do any boat-tailing,you can continue morphing that upper portion closer and closer to a circular cross-section.
I'm modifying my cover to have this feature for the upcoming trip to the wind tunnel.
Professor Alberto Morelli of Turin,Italy says it's an important feature for low drag.
'Kamm' used it on his famous K-form cars
Mercedes-Benz is using it on today's Cd 0.19 concept
Ideally,we'd have a cross-section like this,but it would require that custom bed.
