7 is not a bad number perhaps as it is essentially 1/3. it means significant control but not enough to have 50% majority and far enough from a super majority.

I would personally also consider spreading the rewards at a higher level to the 20-40 slots. I am not sure what the spread is now but if 75% went to top20 and 25% distributed on it might incentivise differently. Have to think more on that one though.