It took me a little time, but I finally realized how to install this bathroom mirror over the tile wainscoting so it’s plumb and hangs at the correct height for our family.
I renovated this bathroom last year for the fall One Room Challenge. You can see the before and after of my renovation in my ORC reveal post here. It was quite a transformation. 
I used subway tile on the walls as wainscoting with pencil edge trim. The pencil trim sticks out past the wall tile by about 1 1/2″. My intention was to end the tile just above the wall mounted faucets. But that resulted in the tile ending right in the middle of the light switches on wall to the right of the vanity(as seen in the picture above). There was no good way to trim around the light switches so I had to install one more row of tile, and therefore had to hang the mirror even higher.
Less than optimal considering this is my kids bathroom and one of my kids is still shorter than me. 😉
It never occurred to me that I could use a cleat (1×4 pine lumber) behind the mirror to bring the mirror away from the wall and allow the mirror to hang straight over the tile. DUH! Why didn’t anyone say anything??!!??!? In my defense, the ORC finish line had me loopy.
So today I set out to make it right and hang the mirror at a reasonable height. I located and marked the position of the studs with this little magnetic stud finder.
I cut my 1 x 4 to span the distance between my mirror hangers, and pre-drilled the holes to make fastening to the studs a little easier and eliminate splitting the wood.
I drew a level line and fastened the board to the studs with long screws. Then I mounted my 50lb. rated picture hangers to the wood cleat. Super simple and super strong.
I reinstalled the mirror and found that the 1 x 4 stuck out past the mirror on both sides due to the placement of the mirror hangers on the back of the mirror. So I traced the shape of the mirror onto the 1×4, took the mirror and the 1×4 down and cut off the corners with a jigsaw. 
I reinstalled the mirror and to my delight, it’s perfect! I can actually see my whole face in the mirror without standing on my tip toes and the bottom of the mirror doesn’t stick out from the tile border like a pregnant belly.
I don’t know why I didn’t think of this sooner, but better late than never.
