Here's my take on it using the AST2 overlayed on a good perspective shot of a 2010 Prius.
It shows about a 2 1/2" to 3" diversion at the spoiler from ideal. I really don't think it'd make a significant difference if you were to just add on to the rear and sort of split the remaining line to try and match ideal.
High Res Version of Above Pic
Like this.
Higher Res Version of Above