Love my helmet mirror. Wouldn't ride in town without it. I like a helmet-mount mirror specifically because I can turn my head just a little and sweep out the entire area behind me.

I'm very nearsighted and can't make out cars at all without my glasses, so the trick is to set it up so that I can see it through the very left edge of my eyeglasses. The light waves have to travel from the object to the mirror, and then to the eye. So, if a car was 100 ft behind the mirror+cyclist, the image in the mirror would look like it was 100 ft + 5 inches away to the cyclist's eye.