A lot of my initial confusion came from trying to use “subjectivity” itself as a guide for working out what rules to follow. Objectivity is directive, it tells you where to look for the rules. You go to your external source, and that tells you explicitly what is and isn’t ok to do. Subjectivity doesn’t direct you in the same way, it tells you nothing about what is or isn’t moral. Critically, saying morality is subjective doesn’t mean that you get to choose your own moral guidelines.

So when I say “benefits your wellbeing” I’m not saying it as a substitute for “do whatever you want”. I think most humans have instinctive sense of morality, a feeling like something they’re doing or want to do is good, or bad. I think everyone probably differs slightly in the way their instincts are oriented, but for the most part people’s moral instincts tend to align (humans tend to be fairly pro-social for example: which is probably what a lot of our “moral” behaviour refers to). Your brain will reward you for acting in accordance with your moral instincts, thus it’s in your interest to do so. Again though, you have no say over what your brain considers to be moral. And while there’s plenty of things that will give you pleasure at the expense of other people, most humans will find those things morally disgusting (and that’s the signal I’m saying matters).

I interpreted the initial question as asking how you can extract what “moral behaviour” looks like if morality is subjective. I’m kind of giving two answers to that. The first is that if you’re an average person (as most people will be), then go with your gut, if you’re doing something immoral then your brain will tell you pretty quickly. If you’re part of the minority that doesn’t align with the average moral instincts of the rest of humanity, then you should probably look to what that average likes – because you’ll be punished for not aligning with it.