Without knowing the exact body/bed combo you have, here's what I came up with. And yes, the 8° angle will do the trick. Reference page 84 & 85 of this.
Truck Aero Paper. I'm not making this up. An Arab from Cal State did his homework.
The conclusion I draw is that at 8°, you get darn near the best Cd, your Lift Coefficient is still very low, AND...You get a ton more space to use under the cap. Best of all worlds.
I figure 8° will get you an 11.5" rear height above the tailgate based on a 75" run and 21" front height. 11° is 8" above the gate.
Also, without knowing quite how to put this lightly, by using flat surfaces and not radiusing the edges, you have 2 strikes against you design wise. The air wants to change directions gradually. By keeping your primary surfaces flat, you introduce a sudden break at the roof to cap transition, the steeper your cap, the more sudden the break, the less the air will like it. With sharp edges, when you create a steeper cap, you lower the pressure on the roof of the cap, the higher pressures on the sides will want to fill this low in, and with no radius to ease the transition, it will tend to create an energy robbing vortex. Again, the steeper the angle, the bigger the vortex. Steeper Bad, Shallow Good. Really look at the chart on the reference paper Pg 85, you see a great improvement at just 5°, so 8° will be a good improvement without risking turning your truck into a massive drag creating vortex generator.
2 other side effects of the cap you'll discover are your truck will be quieter at highway speeds, and (being from Ontario you'll love this) in the winter, your truck will feel like an arrow going down the road at highway speed in icy conditions. I don't know what it is, but my truck felt like it was much more stable with the aerocap on than without in the slippery weather. I think it's the Lift Coefficient going way down that does it. My GMC would always feel like it wanted to spin out at fast speeds with out the cap, and with it, it never slid once.
Hope this helps ya!
Charlie