Hey Joshua, Here's a quick dealy showing your car with the aero template. As you can see, the ideal would be very long. However, when you shorten the boat tail, as you did originally, and appear to want to do again, you pay a penalty. I think you'd be better off staying closer to the ideal curve here and just "Squaring off" the back. I'm sure others will chime in. Great job so far!
I have even cheated the template forward and smaller than it should be, I don't think you'd pay much of a penalty for following the line on my cheat, but if you get much below it, it will not be as ideal. Also pay attention to the transition area, keep it a gradual change, try not to let the air know what you're doing to it.
Here's a cropped pic where it seems practical to end your long Kamm. This would net you around a 40% total ideal, which would improve you from the 15% you're at now, leading to a 40% drag reduction, give er take. YMMV.
I'd only think about messing with the front of your car after you have the back made to your liking, front mods won't return near as much as rear. Of course I'm talking about big front changes, grill blocks and air dams obviously have big returns on investment due to their cheap cost in time & materials usually.