akashic - did you see the teardrop photoshop templates I did in my Kammback thread? If you fit your truck into that shape, with the highest point of the cab at the highest point in the teardrop, you'll be able to see the "ideal" curvature from the tail end of the roof, aft.
I think you're right in that the end point of the ideal curve is going to be higher up than the top of the tailgate.
Here's the image:
