Personally, I think the stock angle on the 2013-2015 Honda Crosstour is almost perfect, nearly the same as the 2000 Insight's near perfect hatch angle.
Even the Civics, though messier, are good enough that it is very difficult to improve without a full boat tail. Unless you want to do a full boattail... don't waste time on the rear glass angle.
There are other areas to improve... like imitating the 2000 Insight's fabulous real wheel well covers! What changes have you made already?