I'm working on a short story set in the early 1920s in England, in which the main characters are friends and collaborators on a large project. One of them is a grand-daughter of Queen Victoria, with the courtesy title of Her Highness. I'm going on the assumption that the other character would use "ma'am" to her in conversation, the same way it's used for present-day female royals...does that sound right (or at least believable?)