Where do you position a mirror in a bathroom in order to see the back of your head? If the large mirror is hung over the vanity, would an oval mirror on pivots be hung behind you at the same height? Lower?
the best arrangement I ever had was with 2 side-by-side medicine cabinets about 3 ft apart, with the mirror hinges on opposite sides, so if you opened both cabinets at once with your head between them, you could see everything... and if you couldn't, just shift one door a bit.
The height was as o'mike said.... my head was centered. I colored my hair with those 2 mirrors for years. was very efficient!