It makes sense to angle the mirror so your rear wing is just out of sight, but the problem is that you are creating an optical illusion. The further you angle the mirror away from the car, the more you are looking to the side and not behind.
I was taught that you don't pull back into the left lane until you can see the headlights of the car you have overtaken. If you angle the mirror too far outwards, the headlight may come into view too early.
The solution is convex mirrors.