I think a lot of times, people just wedge it down and don't pay that much attention to the angle. I have also heard a range of angles ranging from 9 degrees to 11 degrees (and new to my knowledge, your 12 degrees).
I think that the lip at the tail is more for air separation so that the air doesn't swirl as much when it gets to the end of your vehicle.
all that being said, I am not an expert in aerodynamics and am going off of what others have said.
one user on here (basjoos) has done a lot of aero work to his vehicle. I think he used 10 degrees for his rear. I talked to him once about it and he told me he modeled it after the rear of a certain airplane (can't remember which plain either)
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putting a protractor on the back angle of basjoos's car the angle in the photo is about a 22 degree angle and the f150 areo cap is about a 14 degree angle. just wondering what made them come up with these angles?