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In my thought on this, the flow remains attached on this vehicle as it is purported in the initial drawings. These drawings were created from string tests that were done on the car. The main drag I see being created is from the huge amount of "Lift" being induced on the top of and along down the rear window, and a lot of this lift is being created such that it is acting rearward dragging the car back, this is where things go horribly wrong with this shape. So if you have an ideal aero shape, and cut off it's tail, you are better off even if you have a huge flat area on the back of the vehicle, because you are not creating this rearward vector in the lift component that the New Bug is famous for.
So to answer your question, you want to detach flow cleanly from your car when it starts to deviate from the ideal aero shape by too much, this will prevent drag created by lift acting in a rearward vector.
In the Beetles case, find a roof spoiler to attach near the top of the window that follows the curve of the roof remaining lift/downforce neutral.
Something like this maybe:
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