I never liked Hopkins', but Hopkins's is even worse, and Hopkin's is horrible. But how are you supposed to spell something that belongs to several Hopkinses's? This is why I never tried to start a restaurant chain.
It's not a big mystery or even particularly complicated, all regular rules of pluralization and the possessive case. I think people get tripped up in school because they see a specific affectation of dropping the S from possessive forms of the names of some historical personages, e.g. "in Jesus' name."
Edit: As a partially related aside, I have a friend who's right about the same age as me that's incredulous that I was taught to use "they" as a gender neutral pronoun when the subject's gender was unknown (or you desired not convey a gender) back in the late 80s and early 90s. Maybe it's just a regional difference in teaching or something. He's from the UP of Michigan, I'm from Florida. So maybe the same thing is true with possessive nouns