To avoid the blind spot to my perepheral vision from my helmet mounted mirror, I use glasses with a high almost square upper,outer corner. Set up my 3rd Eye mirror to be in that corner. I still have to rotate my head some to see behind.
To those how advocate turning and looking, well not all of us can. I raced years ago and could do the full look without wavering but crashes have altered my balance a little and limited neck motion a lot. I can do the full look when the coast is clear, but I have zero business doing it in tight situations. Mirrors are for me, literally, a Godsend.
Ben