If the mileage isn't panning out, how does it feel? Any difference when you meet/pass a big truck (cattle trucks are the worst, ask aerohead)?
IMHO means In My Humble Opinion: What you have is a wake filler that is only half there. Thinking about a specific angle obfuscates the fact that the ideal, The Template, is a compound curved form. Any angles are merely tangential. Laminar flow ends at about the hood ornament. From there back it's turbulent flow, especially on the downwind side. So you are working with attached turbulent flow.
If the cardboard was cut with curved edges it would flex into simple curves. A curve that starts around the back of the quarter-window and front of the taillight*, as a smooth tangent to the body and curves down and in to the section you terminate at now, should be more effective. The turbulence down the sides (possibly from the front wheel well) would needs some length to reattach. Instead it gets tripped again.
Your vehicle already has a Kamm-back, you're wanting to return it to a partial boattail.
It's a tough problem. I know a solution, but it would be painful:
One of my favorite cars, the Volkswagen Golf GTI w12 650. The Golf is a Kamm-back, like yours, but this has been converted to a bubble-top coupe wit only the skeleton of the original top left. That's what it takes. It has a 200mph top speed.
The wheelwells and underbody (diverter and difusser) offer as much opportunity as your 30% tail.
* I know, I know.