wellywales: first of all, left over points means nothing, so don't be afraid to use them. 2nd the ratings works simmilar to other games with the exception that an average is taken from all the people you beat vs. all the people that beat you. so you do not have to win to gain bkr. It all depends on the BKR of your opponents and their finish vs. your bkr. I know that is a simplistic answer but it really hasn't ever been explained much better.
Bottom line-WIN, and if you can't win, finish in the top 5