The problem with applying a tournament ratings system is the following:

In a tournament you are always trying to win each game, and that will increase the chance of winning the tournament.

In a pond you can decrease the likelyhood of winning while increasing the likelyhood of finishing top 5.

Since the position at each round is proportional to the likelyhood of winning, I recommend that position at each round be taken into consideration. That way being bottom for 10 rounds is not rewarded (unless you catch up at some point).