I don't think you are going to find many people willing to post what they use exactly.
For me, its a combo of:
yards in the half by each team
score differential
turnovers
injuries
yards per play
recent trend (2nd q and 2half of 2nd q)
coaches (do they hate each other, do they like to run up the score)
Lines seem to have been moved to0 low recently. I play a LOT of overs and a small number of faves when the yardage is like 350 to 70 with a line of under -7. I also play the occasional dog when the team had more yards in the 1H. Up till last year, I mainly played unders.
I'm going to begin posting my plays as I did tonight.
In tonights game, the reasoning was:
Toledo could not stop N IL at all on their last 3-4 drives. I figured they were good for at least 17-21 in the 2H. If Toledo could score once we'd get the over either because they got more competitive or got a garbage end of game TD. In terms of the side, half time yards were like 270 to 70 or so (I can't recall) and to me that should be -7.5, not -4 assuming the two coaches are likely to pile on points which is what we always see in these mid tier conferences.
In the NFL whether home or away is winning has a big impact on totals though not as much this year as in previous years. My NFL 2Hs have been a wash this year where as it was great for a few years.
Another thing worth noting is in CFB they will never hang a line over 31 or below 20.5. Last weekend, MI/IL should have been something like 40 or even 42 per my opinion. They hung it at 30.5, it incredibly moved all the way to 32, which you NEVER see. With overtime they scored like 50 or 60 or so. The over in the 2H was my biggest bet I've ever played.
Some other examples of great overs hung at 31 were East Carolina and Oregon. Neither one had any business being below 35.
Sharps are trained when they see 31 to bet under and the books know they can't put up 35 or they get one side action.
-Sean