Hi Phil,
I did a quick drawing of "The Template" over a web image of a 2011 Prius.
Some things to note:
The Prius back glass drops quicker than the plan taper.
In order to make up for it, I shrank the template a bit and lowered it to put it in a sort of compromise position that you see in the image.
I did this because it would seem to me that once the air starts moving along a given taper, it would be unwise to push it back up too much, I'd think a sort of go with it idea would keep the air moving as smoothly as possible.
The black line was used as a measuring device, it appears to me that a trailer would end at 82" or so if this line were followed.
Anyway, hope this helps.
Charlie