angle would be hard to say,well for me anyway. Speed,where its located(clean or dirty air) ,wing design and how much downforce is needed.All have big effect. I would start low maybe 2 to 4 degrees and move up a degree at a time and see how it effects handling,et and mph. Remember more angle more drag don't go so far that your slowing car down with ton of drag. On well designed wing theres a break point where drag goes up miltiple to downforce achieved.On my wing on the TA/D after 8* drag went up big time but downforce increase was about a 1/4 of drag increase. It also had down force at negative 2*.
We where running Alcohol Funny at Darlington and car was moving on top end.Added 3/4" lip at 15* to deack and car went straight as arrow and actually picked up cpl mph. It don't take alot at times.