As a John who goes by Jack, I’d have to say JFK for that dynamic. It’s my go-to when people act embarrassingly confused as to how that name came to be. My dad is also John, so, starting with my parents, I’ve been called Jack my entire life, but people act like I went from John to Malakai until I mention JFK, and then it clicks. This works better with older folks that remember JFK being called Jack Kennedy, but I’ve found that people that aren’t aware of that trail off anyway because they don’t want to say that they didn’t know that bit. I hate having the conversation in general. Like, I didn’t ask why your name is Kayleigh-Lynn because I don’t want to highlight your mom being basic as fuck, don’t act incredulous about the crazy concept that John can be Jack even though it’s the same amount of letters.