You can use any formula you like, as long as first place gets the most and 3rd place gets the least. And the total can't be more than the total prize money. So you could have 50% for first, 31% second, 19% third. Or it could be 60% first, 30% second, 10% third. Or give the winner 40%, second place 30%, third place 20% and send me what's left over.