'Best' here means most advantageous to the agents involved - both the focal agent, and the other agents who join in the convention. What I have in mind here is something like the Nash Bargaining Solution, which is both unique and (in some sense) optimal.

"Morally permissible" by the best code, which is also the code mentioned in the first paragraph. What I am claiming is that the 'moral should' can be reduced to the 'practical self-interested should' if (and it is a big if) it is reasonable to treat the other agents as rational and well-informed.