Dear Brendan,

You’re asking the wrong question. According to your formula (which is, of course, academically correct but no use whatsoever in creating or maintaining a style book), we shouldn’t apply to the polis for a verdict; we should merely observe behavior. It is not a question of “What do you prefer?”, but “What do you do?”

This is, as you may have guessed, an argument for s’s. Say a writer had the last name Polis. People would say Polis’s books. This pronunciation conveys information, which people like to do when speaking. Better to be clear, and better to follow usage.

It’s “s’s.” The people have no say, except insofar as they speak.