From the Porsche 914,you can see what Lotus did with their Europa,with boot height and length pushing the rear out near the 'Template' for Cd 0.29.
If your Kamm addition filled in this compromised area,you'd be poised for a nice drag reduction.
You can see how the 1st-gen Golf/Rabbit would have the lowest drag if it's roofline had been 'filled in.''
You can also see how Chrysler lowered their Charger's drag by filling in this area.
A quick cardboard mockup would give you your fuel savings numbers.