Hey Mistareno, the 22° angle is not the goal. If your vehicle is not long enough, you cut off where ever you are an it'll work well. If you make the 22° thing your goal, you'll probably end up sloping too quickly and end up with a less efficient design, that you can't put as much stuff underneath, sort of a lose-lose deal. I looked at what you've drawn with a template overlay and it looks about perfect if you follow the template. The top line you have drawn is at about 12° overall slope, right in the sweet spot for best efficiency.
If you wanted to raise the back up about 5 inches (red line) and drop 10° (blue line) overall, this would only affect the Cd very slightly but I think it would make your Lift Coefficient drop a lot making the vehicle feel much more solid on the road.
I base what I'm saying on these charts which were in a detailed paper on pickup truck aerodynamics.
