Rock,

I adjust the ratings after each week to use on the following week's matches.

For example, last night's match:
Kangaroos rated on 102, Collingwood rated on 90, No home ground advantage bonus, Collingwood head to head bonus of 6 points
Predicted final margin: Kangaroos by 6 points

If I had simply used the same ratings that I had at the start of the season, I would have had Collingwood coming out on top.