I sell tankless water heaters all over the world, but in real life, I'm an electrician.
I just wired and installed five of em in a customer's remodel job, and that's exactly the height she wanted them.
I don't think there is a code pertaining to sconce height. "A little higher than EYE-HEIGHT" always seems to be just right. And it's much easier to change bulbs at that height, plus there is a better light spread downwards as well as upwards.