I bought one of these when I went from a hybrid with a side mirror to a road bike where I could not mount a side mirror. It did not work for me on a helmet as the helmet can be in slightly different positions and there is some vibration. I put it on the left side of my sun glasses. If you look with your left eye and can see the very top of your shoulder when you lean forward (as you would be positioned on your bike) and the very edge of your left ear, the mirror will be in the right position to see traffic and whoever is riding behind you.

I also discovered a really unusual thing. My left eye is very near sighted and I did not know if I would see anything behind me. But since the mirror is so close to my head, I can see what's behind me very clearly. Now I feel like I riding blind if I don't have my mirror.