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.