For heaven's sake put a big old piece of masking tape like a flag on the little tiny allen wrenches that come with it!
That way, when it sticks in the little tiny allen headed scew while you're doing the fine tuning at the range, and you drop it on the floor, in dim light, amid hundreds of empty casings, you have a half a chance of maybe finding it.
Don't ask me how I know.
As above., they are "sorta set" at the factory for 50 feet. But other than Queen Elizabeth, I don't know anyone with a hallway that long. So figure the distance you'll most likely need it, "in extremis", and adjust accordingly. 21 feet or so sounds about right to me too.