If I understood correctly, you say some users get the number of merit needed to raise the rank much faster than their activity allows them to move higher. And you propose to apply some formula that would allow to increase the rank depending on the merit, not on the activity indicator. If this is what you meant, then I have a question: why do we need activity at all? Maybe it makes sense to remove it altogether and leave only merit, on which your rank will depend? Those who write good posts will move faster, and those who aren`t able to receive merits will remain standing still. Now the situation is such that the activity either inhibits the participant in obtaining a new rank, or doesn`t play a special role, since practically noone is looking at this indicator. Activity has ceased to be informative. Merit has more to do with rank than activity does. The only thing that it indirectly demonstrates is the time spent on the forum, but this can be found out by looking at the date of registration of the participant.