Just remember having attached flow doesn't necessarily mean low drag, the case in point is the VW beetles which has excellent attached flow but a woeful Cd of 0.48:
![](http://us1.webpublications.com.au/static/images/articles/i1086/108675_12lo.jpg)
If you are getting attached flow and the angle is more than the 15% from horizontal it may be worth heat bending the trailing edge back to horizontal like the rear spoiler on the prius:
![](http://www.twoglasses.com/images/prius.jpg)