formula for strength of schedule?

So I'm trying to create a "strength of schedule" component for the football season. Having trouble thinking of a way to adjust it to reflect the average value of a team's opponents up until the current day's games. I've attached a small spreadsheet attempting to show what I'm trying to do... I manually filled in the values for "b" team's strength of schedule with an explanation of how I got to that number. Any ideas? Thanks!