Lostcause is bang on, as usual.
To reduce drag on a hatchback, your spoiler mustn't extend upwards of the imagined line/arc continuing the roof line, in profile view.
These Volvos are a good illustration:
With their C30 "Efficiency" model, among several aero enhancements, they
reduced the rear spoiler angle from the regular model, essentially turning it into a roofline extension (extending the slightly downward slope):
(
source)
vs. original model with higher cD:
