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Yeah, I know what you meant. You wanted to get the last bit of the boat tail sloping 22°, again, that is not the goal, it is a result of a vehicle being long enough to reach the 22-23° slope at the end. If your vehicle can't be that long, you are faced with having to make a compromise. The place to make this compromise is certainly making it shorter in back as has been shown time and again.
You make the biggest gains by staying at or a bit above the template line until you run out of vehicle to support the curve. The red line I drew above has a final angle of ~18° at the back. If you made a 22° overall angle boat tail, it would be a disaster aerodynamically.
You really really want to make the shape at the front of your add on to be as close to the curve as possible. Making your rear cross section smaller would be a bad thing here, I know it is counter intuitive. The goal is to upset the air as little as possible as you drive through it. By following the template to the end you do that, if you tinker with it, you will invariably create more turbulence which will erode your Cd gains even though you have made the back as small as possible.
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