Without addressing the specific details of the formula (which I currently have no opinion on), be sure to include how often the check is checked in your consideration. Is it rolled once for every time a creature dies (i.e. one roll for a ten attack dragon, ten rolls for ten one attack skeletons)? Once for every power gained (ten times total for both dragon and 10 skeletons)? Once a turn? I have no strong opinion on which option, and all of them can produce the same overall end result just by making sure the formula is accounting for, eg, six rolls when gaining six power. Once per death seems like the obvious thing, but it also means that it is very heavily weighted towards killing few big creatures as opposed to killing many small creatures.