If you have some concept of "3^^^3 disutility" as a tractable measure of units of disutility, it seems unlikely you don't also have a reasonable idea of the upper and lower bounds of your utility function. If the values are known this becomes trivial to solve.

I am becoming increasingly convinced that VNM-utility is a poor tool for ad-hoc decision-theoretics, not because of dubious assumptions or inapplicability, but because finding corner-cases where it appears to break down is somehow ridiculously appealing.