OK so I came up with the idea that the volley number for each weapon is its listed range divided by 60 rounded up. Giving these numbers: Volleys per weapon

But I just came up with a much simpler way to get pretty much the same result.

One hand attacks (e.g. darts, hand crossbows, spells): last/closest two volleys normal Two hand attacks (e.g. bows, crossbows): last/closest two volleys normal, 10–3 volleys disad

Exception: spells w range 300 feet or more. (Eldritch spear, Meteor Swarm.) There are so few spells with this range. It's worth making exceptions for them.

Of course sharpshooter feat removes disad here