Your words have many connotations. 'Subjective' and 'relative' are often misunderstood at the best of times. If you could taboo these, we'd see where any real disagreement lay.

Also, I don't see any more disagreement here. 'Greatest good for the greatest number' is a calculation to be made. If that statement sums up all of ethics, then it is a logical fact, not a physical one. I can't shoot your fact to make it different. We can simply turn the criterion for 'good' into a computation, so that we input physical facts, and it comes out with advice on what to do next.

Even if this morality isn't grounded in the 'nature of the universe', if this is all that we care about, then the computation is still a logical thing(y). Even if evolution adapted us to desire this, if this statement is the summation of all ethical facts, then that wouldn't change the computation. Which computation we're interested in is a product of contingent facts, physical, evolutionary ones. This doesn't change the fact, that, when we compute the greatest 'good' for the greatest 'number', we're talking about a computation that's substrate neutral. And logical.