If I may,
What I think Aerohead is trying to say is, once you get past the front edge of the rear glass, you drop below the template. In doing so, the air cannot remain attached beyond this point due to the curvature of the car dropping away faster than the air is capable filling in.
The 23° limit is the absolute limit if you follow the template out to the end. Anytime you exceed the template curve before that point, you exceed the ability of the air to remain attached.
That said...It's my turn to muddy the water.
I think the flow attachment thing happens in degrees, and the Template optimizes the flow attachment so the pressures are at their best to achieve the lowest Cd. I think the flow remains attached beyond the top of the rear glass, but it is under "stress" and the pressures create a lower than optimum Cd, so it's still good, just not great. More like a grey area is created when you are sub template, until you reach 23°....then it is done, shredded, no more grey area, its over.