The problem with the "highest profit percentage on turnover" that you suggest, if I understand it correctly, is that say for example someone gets lucky and wins a bet with a very high profit percentage, i.e. 700% ROI, s/he might not want to place additional bets during the competition period to retain the high ROI and the high position in the competition, which would be the correct strategy as far as the competition goes. This in effect will turn the competition into "Who wins a bet at the highest odds".
On reflection, you are right.
Leave things as they are.