I don't rely on mirror position or anything.
I move my head around while looking in the mirror when I plan on changing lanes and or turn in a road.
I will also look over my shoulder left and right as well as look in the mirror. (one reason I prefer 2 door cars, they have no B pillar or the B pillar is further behind.
1 Blind spots in the mirror : you can't fully eliminate em if you don't move your head around
2 Modern cars have such big A pillars that you can have a small car, cyclist hidden behind the left A pillar , it if he's at the right speed vs your speed while approaching the intersection..
Moving your head around avoids all that, because your mirror view will move around massively.
