A team's point differential is the difference in points scored and points allowed and divided by 16 games.
I have 9 years of every NFL team's point differential categorized by the number of wins they had for each of those seasons. You can download them for free. The charts will show you what a 15 win team down to a winless team will average winning or losing by.
If you want to create your own power ratings, you want to make a scale to measure each team's strength. Think of the NFL as a 24 hour clock. You want to find the average to start. A clock has 24 hours so 12:00 would be the average. Since a team plays 16 games, then 8 wins would be average.
Now we have to figure out how many games you think each team should win. I have all the team schedules in the book highlighting the short weeks and the 10 day rests. There are early starts for west coast teams heading east.
There are several other stats that could trip a team up from getting their potential wins for the season which gives you a good idea of how a team will fare this season. Once you have determined the number of games a team will win, you can assign a power rating to them.
0 wins- only one team Cleveland point differential -11
1 win- just two teams point differential -14
2 wins- point differential -12
3 wins- point differential -9 has dropped to -7 last 3 years.
4 wins-point differential -8 has dropped to -7 last 3 years.
5 wins- point differential -6 has dropped to -5 last 3 years.
6 wins- point differential -3
7 wins- point differential -2
8 wins- point differential 0
9 wins- point differential 2 has dropped to -1 last 3 years.
10 wins- point differential 4
11 wins- point differential 5 went up to 6 last 3 years.
12 wins- point differential 7 has dropped to 6 last 3 years.
13 wins- point differential 10 has dropped to 9 last 3 years.
14 wins- point differential 11
15 wins- point differential 12 just two teams.
Each season always brings in new rules which is starting to get ridiculous. This year the helmet to helmet rule can get your player ejected. Defensive backs could become the most valuable players on the field. This will be something to track and see how many wins this could cost.
This rule looks to have the most impact of any rule they have added to date. There hasn't been as many teams going for two points, but we have suffered a missed extra point or two. The worst is determining a catch. They are not in sync when making a call that determines the outcome of a game. I just pray they don't get to crazy with ejections this year.
Injuries can effect the point differential of a team and therefore the number of wins in a season. They will effect power ratings to a certain extent so adjusting for a key injury can be critical to beating the spread.