Happy New Years, Mearls! Great to see you here again and good luck with the coding!
I've leaned into the "art" side of combat design (I'll attach the cheat sheet I use for guesstimating difficulty of fights), but I always appreciate it when someone takes a hard look at the numbers. I'll definitely watch for what you come up with.
Are you committed to defining difficulty by % HP loss expected? Or did you consider including other resources expended there? I know I've had a conversation with my players after a combat where we had differences of perspective about how difficult the fight was and it came down to me as GM focusing on HP loss / downed PCs, and the players being a bit more focused on other resources expended.