Part of my preparation each week during the NFL season involves figuring out what the line "should" be and comparing it to what numbers Vegas is putting out there. Today, I'm going to take you through where that "should" number comes from: NFL power ratings.

You've probably seen plenty of power rankings of NFL teams -- Pete Prisco does an excellent version of this on this website each week throughout the season -- but the slight difference here is translating those rankings into something actionable for NFL point spreads.

How does it work? First, you start with a team that you think is perfectly average in 2018. For me, that's the Lions, a team that can beat up bad teams but struggles against playoff contenders. As a result, I have the Lions with a zero rating, meaning they're an average NFL team to start the season.

From there, you figure out which teams you'd make a toss-up against the Lions on a neutral field and give them zero ratings too. For me, that's the Seahawks, Texans and the Panthers, but you might grade these teams differently. From there, you figure out who'd be a slight favorite against an average team and give them a rating of 1, and do the same for a slightly below-average team and negative-1. 

Below, I've listed my power ratings as of Monday coming out of the third week of preseason football. I've also included a few caveats, like how injuries affect my Week 1 ratings and other nuggets.

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Week 1 power ratings

TeamWk 1

QB injuries: You have to account for injuries to quarterbacks and a handful of other key players when setting your ratings each week. No, Eagles fans, I don't think your team is the sixth-best in the NFC, but it's looking like there's a real chance Carson Wentz could miss Week 1. Even as a Super Bowl MVP, Nick Foles deserves a downgrade from Wentz, and once the starter is back I expect to see the Eagles at the top of my ratings. The Buccaneers are also dealing with a missing quarterback in the first three weeks and have a rating that reflects that.

Other absences: Starting QBs are going to cover most of my ratings adjustments from a team's true value throughout the season, but there are some instances where I'll give my ratings a slight push one way or another if a team is facing absences at other positions. For instance, the Cowboys are missing their starting center and could be without guard Zack Martin as well, though it looks like he's trending toward being available. I'd have a lot less confidence in the Cowboys offense with multiple linemen missing. Baltimore is going to be without star corner Jimmy Smith for four games as well, and I've adjusted them slightly downward as a result. Doug Baldwin's absence would be a hit for the Seahawks, but it looks like he'll be available.

These ratings are obviously subjective, and yours might look quote different if, for example, you're high on the Giants or completely buying in to the Browns before they hit the field in a real game. Now, let's take a look at how these ratings apply to the Week 1 spreads.

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Finding line value in Week 1

HomeLineAwayPR HPR AHFAPR LinePR Diff

In the table above, you'll find the home and away teams along with the line for the game, then the power rating of the home and away teams, then my home-field advantage number as described here and finally what the power ratings and HFA say the line should be as well as the difference from the current line.

So let's go through the Thursday night opener as an example. The Eagles are favored by three points against the Falcons, a line that I believe reflects the possibility that Carson Wentz won't play. Without Wentz, I have the Eagles with a two-point rating, while the Falcons are a three-point rating. That means that the Falcons should be favored by one point on a neutral field. However, the Eagles have a four-point home-field advantage figure, as their defense has been fantastic at home over the last two years. Adding four points of HFA gives me a power rating line of Eagles -3. Since that's what's on the board, there's zero difference from my power rating line and zero value to be had.

One note: Once a line gets above a touchdown, I reduce the favorite's advantage to account for potential garbage-time scoring. Teams that land between -7.5 and -10 get knocked a half-point, while anyone favored by more than 10 is reduced a full point. So if you do the math on the 49ers-Vikings matchup, for example, it comes out to Vikings -9, which I then dock a half-point to make Vikings -8.5 for my power rating line.

Speaking of that 49ers-Vikings game, you'll see it's one of three games on the board where I think the line is off by at least two points. Both the Vikings and Ravens should be favored by more, and I think both make for value plays based solely on this method of handicapping teams.

The other game that has nice value to me is Redskins-Cardinals. Vegas is currently making that matchup a pick 'em, but I think the Cardinals are only one point worse than the Redskins, and since they get three points for their home-field advantage, I'd put that line at Cardinals -2. I have a feeling the Cardinals are going to be a team that offers value often during the first half, as I don't see them being anywhere near as bad as futures and win totals would have you believe.