Gene, I've been wearing glasses since I was a teenager, as I'm pretty nearsighted.
I bought an Optivisor when I got back into modeling in my early forties. I've gone from a 2.5 set of lenses to a 4. I wear it over my normal glasses.
My understanding is that the number is the magnification factor divided by two. The main thing I found when picking lenses is to determine the focus distance. With my glasses and the 4 Optivisor on, thats about 10" in front of my nose. Thats comfortable for me, but I would not want it any closer.
Yes my derm wears them when shes cutting pieces off of me. It's pretty funny. She's a real cute little lady, when I go to see her shes got the lab coat on, the pearls the leather flats. Goes out comes back in in clogs, a gown and that optivisor on. Yikes!