I was doing WAB before the NCAA was (I called it WAR). Basically I calculated the winning percentage of a bubble team against a sampling of teams based on their Pom rating and whether the team was home, road, or away. If you played a team that a bubble team would have a 70% chance of beating, you would get a .3 WAR for a win and a -.7 for a loss. A team that a bubble team has a 30% chance of beating would earn you a .7 for a win and a -.3 for a loss. WAB does the same but their calculations for the percentage a bubble team has beating an opponent uses a different formula.