The so-called “Golden Rule” of “Do unto others as you would have them do unto you” is my go-to.
Granted, it doesn’t always work. Like, something I would want someone to do to/for me, isn’t necessarily what another person would want.
Still, it’s a reliable rule because I can always say I did my best (assuming I had no other information about what the other person/people would want). So it helps short-circuit regrets and second-guessing.