And we're rejecting the idea that F is simply the strength of my preference for my partner's satisfaction. If that were the case, there'd be no problem calculating a result... though of course no guarantee that my partner and I would calculate the same result. Yes?

Yes. If you and your partner agree- that is, A/B=C/D- then there's no trouble. If you disagree, though, there's no objectively correct way to determine the correct action.

Going back to the original statement... "an ethical rationalist's goals in relationship-seeking should be to seek a relationship that creates maximal utility for both parties" seems to be saying F should approximate 1. Which is arbitrary, admittedly.

Possibly, though many cases with F=1 seem like things PhilosophyTutor would find unethical. It seems more meaningful to look at A and B.