The accepted height is something like 1-3 feet off the floor. Too high and the heavier birds can hurt their legs leaping out of it. Too low and you may as well not have them at all, allowing the birds to lay on the floor. Most of this has to do with convenience for you, the chicken keeper. I dont believe the chickens dont care
For discrete nest boxes, I suggest you place the nests directly behind the roosts, along one wall, at the same level as the roosts. Alternately, place the nests just below and in front of the roosts. This way, the nests are situated where the birds can go from roost to nest quite readily and this may encourage them to lay early.
For a communal nest, make it about a foot off the floor.
It's nice if you can cut a hinged door to the outside along the back of your nest(s) for collecting eggs from the outside. Again, easy on you and you don't need to bother the chickens while they're in their "sanctum sanctorum."