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Scoring system as August but your top 5 scores count so those who can put in volume can maintain interest. (Bit like the old DtD league). ...
If you have just have this it can purely come down to volume - the people who will do best or who win will be those who can just keep on playing until they rack up the 5 wins (or however much it needs) to overtake everyone else.
Obviously having people play more is good for Sky, and like you said it can keep people's interest for longer.
The numbers can be played around with, but I think a better way of approaching it would be to have something like a minimum criteria of 10 scores - but a bonus criteria that if you play over (for example) at least 30 MTT's you get to discard your lowest 10 scores.
That way people are given a reward for playing on the sponsor more, but they still have a set of results which will be large enough to average out and not help them
too much compared to the people who only play 10 or so.