Not sure if directly related, but some people (e.g. Alan Carter) suggest having indifference curves. These consist of isovalue curves on a plane with average happiness and amount of happy people as axes, each curve corresponding to the same amount of total utility. The Repugnant Conclusion scenario would be nearly flat on the amount of happy people axis and the a fully satisfied Utility Monster nearly flat on the average happiness axis. It seems this framework produces similar results as yours. Every time you create a being slightly less happy than the average you have a gain in the amount of happy people but a loss in average happiness and might end up with the exact same total utility.
Yep, I've seen that idea. It's quite neat, and allows hyperbolic indifference curves, which are approximately what you want.
It occurs to me that the various utility indifference approaches might be usable in population ethics.
One challenge for non-total utilitarians is how to deal with new beings. Some theories - average utilitarianism, for instance, or some other systems that use overall population utility - have no problem dealing with this. But many non-total utilitarians would like to see creating new beings as a strictly neutral act.
One way you could do this is by starting with a total utilitarian framework, but subtracting a certain amount of utility every time a new being B is brought into the world. In the spirit of utility indifference, we could subtract exactly the expected utility that we expect B to enjoy during their life.
This means that we should be indifferent as to whether B is brought into the world or not, but, once B is there, we should aim to increase B's utility. There are two problems with this. The first is that, strictly interpreted, we would also be indifferent to creating people with negative utility. This can be addressed by only doing the "utility correction" if B's expected utility is positive, thus preventing us from creating beings only to have them suffer.
The second problem is more serious. What about all the actions that we could do, ahead of time, in order to harm or benefit the new being? For instance, it would seem perverse to argue that buying a rattle for a child after they are born (or conceived) is an act of positive utility, whereas buying it before they were born (or conceived) would be a neutral act, since the increase in expected utility for the child is cancel out by the above process. Not only is it perverse, but it isn't timeless, and isn't stable under self modification.
What would be needed is a natural, timeless zero for the act of bringing a being into existence. Something that took into account things done before the being is created as well as after. A sort of Rawlsian veil of ignorance about whether the being is created at all.
This suggests another approach, vaguely derived from utility indifference and counterfactuals. What if the agents "believed" that being B wasn't going to be successfully created? If they were certain that its creation or conception would fail? Then they wouldn't be, eg, buying rattles ahead of time.
It seems this could define a natural zero. If agent A decides to bring being B into existence, the natural zero is the expected utility B would face, if agent A expected that that B could not be brought into existence.
Then, given that agent A actually expects that B will be brought into existence, they can freely buy them rattles, or whatever else, either before or after the birth or conception.
What do people think of this approach? Does it solve (some of) the issues involved? Could it be improved? (yes)
Though I hope this approach is of use, I'm personally not enamoured of it. My objection to total utilitarianism is entirely to the repugnant conclusion. I feel that bringing a very happy being into existence should be generally a positive act, and prefer systems that weight utilities to prevent repugnant conclusions, rather than ones that zero-out the creation of new beings (there are several ways of doing this).