[SvdM]

Okay, so I can assume 12° is the 'no fail' amount for DIY jobs that's still good for highway speeds, say 120km/h? I'm assuming the 18° works if your handiwork is tidy and you are a little morecareful with your right foot

So I can postulate:

1. That if you're going for the 'chopped off' boat tail design, best go for 18° rather than 12° in order to keep the low pressure area at the rear to a minimum.

and

2. Even if there is some detached flow on your 18° boat-tail, it's still better than no boat-tail at all (?)