I was reading Why does the ISS atmosphere contain nitrogen, and I wondered if some of the explanations there would also apply to a base on the Moon. There are considerations on the Moon that don't apply (so much) to the ISS.
A lower pressure atmosphere is easier to contain. It doesn't require that as much stuff be shipped up from Earth - especially Nitrogen which is not available there, while at some point Oxygen can be obtained from lunar soil once the infrastructure exists. The larger a base gets, the more this becomes a factor. Also the larger it gets, the easier it is to install the proper airlocks so that people arriving and departing can transition between the atmosphere on the ships and that in the base.
Isn't there wiggle room where the oxygen fraction can be raised, the nitrogen fraction can be lowered, and the pressure can be some substantial fraction of Earth sea-level so that transitions between base and ships aren't difficult? Can it be said what the best mix would likely be?