When creating you own line or ratings, understand the need to deviate from averages based on matchups.
For example, if two high scoring teams meet each other, know that the sum of the individual parts will not be equal to the whole, they will be less.
If two average teams can be added up to create a total, then two high scoring teams will do more than add up to create a total. You must enahance that number. Often you will see it in the marketplace. That Total isn't inflated, it's just that two teams that run and score or shoot three's like mad will accelerate the number of possessions for each team, often increasing the score.
Say you have a simple 1-10 scale of teams scoring, with 10 being high scorers and you can add the tatings to figure a Total. Two 5 teams could meet and produce a 10 score.
But two 9 teams that meet will not produce 18, they will produce something higher. It's the nature of the beast.