All these supposed pro betters basically underperformed me last season. If not for the 49ers having a QB apocalypse I would have had a perfect playoffs, just as an example.
And of course the baseline is looking at last season. You start off looking at the team was and then look at what changed to see what got better, what got worse, and what is uncertain. For example lets look at Cowboys vs Giants just to demonstrate this in principle.
The major changes from last year for the Cowboys on offense are losing a mediocre TE, moving on from Zeke who Pollard had basically replaced as the year went on, and signing Cooks who is a number 2. You could also add in their other WR Gallup is now 2 years off an ACL injury and players tend to look better 2 years removed versus the next year and meanwhile the offensive line is staying the same with the core players returning and their rookie LG becoming a 2nd year LG so he will statistically improve so their offense can safely be called better. Meanwhile on defense the only major change was replacing a scrub CB with Gillmore so that defense is certainly upgraded even if he lost a step.
Now for the Giants they basically held serve. The big move they made was getting Waller, but otherwise they largely held steady. I like their draft a lot but I am not going to put much stock in rookies week 1. That is a good way to lose money quickly.
Therefore when I look at both teams I see that the Cowboys, who again covered this spread last year, improved by more and addressed more of their positions where they could improve so I just do not see how the gap shrunk. If anything the gap grew which means I would expect the Cowboys to be able to cover an even larger spread.
I could do the same for the rest of the bets I stated but that is my formula. Start from last year and then look at the signings and try to figure out how much uncertainty I have about some of them. I have a lot of uncertainty about the Browns for instance because I have no idea if Watson was just rusty or genuinely washed up. I do also wonder if the offense just does not play to his strengths but again that is why I have too much uncertainty to touch that game until some more data appears.
Edit: I am now also happy I got in on that Chiefs line when I did.