Q. I’m having a difficult time finding a rule that governs afterthoughts. For example: “I told him I would pay my respects another time, if necessary.” Comma before “if”? Seems like there should be.
A. Anything intended as an afterthought should be preceded by a comma, or by some other mark of punctuation—a dash, for example (or parentheses). A period could also work. In your example, the comma before “if” is the only thing that tells readers that “if necessary” is an afterthought. So keep it, assuming that’s what you intended.