When to Use First Names

I'm presently dealing in my police procedural with when to use first names and when to use last names. In some published works I have found it to be confusing when the author jumps back and forth between first and last name, especially in non-dialogue passages. (Huh? Who's that?) I finally made myself a sort of formula. I use the first names of my police characters only when someone they know well addresses them directly. Otherwise, all police are called by their last names without any honorific. Non police people (suspects, witnesses, etc.) always get an honorific when addressed directly, but just the last name without an honorific when spoken of outside their presence. The novel contains a group of homeless people who identify themselves to each other only by first name, and so they are called by their first names in every situation. Thanks for reading. Joan Sween