I think 6 feet from the floor is a standard. However, I think some people like sconces at face level but I don't - it bothers my eyes or something. I find that just above (my) head level is better, no problems with shadows or anything with the ones I chose.

I have a two-sink vanity. My light fixture (that is the glass shade) will "end" two inches above my mirrors, but I'm wondering if I should drop my mirrors another two inches. My medicine cabinet mirror is 20"by 26" long, which is really not long enough but it's the best I could find to work with everything else.

So my three light fixture, which is 24" wide, is much higher than 72" off of the floor, it's more like 78" off the floor.