Frankly, I'd say the only way to do this is to write what seems to fit, and then go back and look at it as objectively as possible (if you have trouble with this, get someone whose judgment you respect to do so) and ask: is this part of the story? Does it add to character/plot? Or does it seem forced, tacked on, unnecessary. Read it as a reader, not as the writer. If it seems to fit with the story as a whole, then it fits. "Self-indulgence" only enters if you don't give it that consideration, but insist on something being there when it simply doesn't belong; when it doesn't grow out of the story's natural development.