Shamelessly plugging my '03 Tacoma Prerunner thread...
Check out the exchanges between Aerohead, MetroMPG, and I about fitting my truck under the ideal curve and chopping the wake to deal with my short-bed. You could try the same with your truck.
The slope of the ideal curve averages about 10-11 degrees on the conservative side. Above the bed the curve is pretty straight -- it doesn't really get a lot steeper until it's past the tailgate. A rule-of-thumb you can follow is that you want it a tad shallower than 1-to-5.
That's not to say that you can't experiment with curving a wee bit more aggressively -- a couple guys on here have demonstrated attached flow (through tuft testing) with a curved "fastback" approach like in your sketch. But if you're sold on the rear-window idea (which I used as well), then you might simplify to a straight back. I personally like the way a straight back opens you up to curving in the side-to-side direction.