What do you mean "a flush wall"? (bathroom?
Mount it on an inside wall for two reasons, it is not affected by the cool temps of the outside wall, and there is usually no insulation in the wall to make fishing a wire difficult. So, pick out a good spot on an inside wall. Mark 58 inches to the center of the thermostat. Try to locate it next to a stud. That way you can secure to the stud and still have plenty of room to chase the wire through the wall. Go down stairs and make sure there are no wires blocking the way, then upstairs drill a tiny hole right next to the wall. If it's carpet, good! If not, make the hole small enough so it won't be noticed. Go down stairs & locate the tiny hole, now you can measure and drill a wire size hole up through the stud. ( If you're as unlucky as I was you'll find some idiot (me) put an electric wire right there.) Now drill a wire size hole where the Coal Trol will go. Fish the wire up, mount the 'stat and voila, you have a hidden wire.