No matter where you situate your mirror, close-field or on your handle-bars, your eye will be focussed on infinity. You should not be looking through the close vision part of your bi-/tri-focals. I tend only to wear a Take-A-Look mirror when leading or riding shotgun on group rides, but only to observe participants' progress.

I think body language is a major contribution to safe riding. It must be demonstrable to traffic coming up behind you that you are aware, not to put the onus on you for for your own safe conduct, but to establish a connection, a bit like looking into a driver's eyes at a road junction.

Okay, as I've got older looking over my shoulder has got a bit more restricted, but I find that leaning forward and looking back and down, past, rather than over my left shoulder, enables me to properly assess the road behind and establish that I'm a sentient being to approaching drivers.

Whatever you do, do not depend on the limited field of view available through any mirror.