it depends a LOT on the matchups. I use distributions that work better in baskets with more points scored, but they are pretty good in predicting who wins by what as a percentage. A couple things that I also look at are historical point differentials between the two teams, point differentials to other teams, fg's made/attempted vs TD's, playing styles (run/pass), coaching philosophy. It all adds up to whether or not I think the team wins by exactly 7 (I dont middle enough to care much about the 6).
If I really liked the team to cover the 6, I would open team A -6 for 1.5%, if the line moved to 7 or showed up as a 7, I would look closely to how significant the 7 is to the matchup. The actual book that hangs the 7 is very important. If it's a jackass book like SIA, then it's not very significant, they inflate their lines with the public (and they arent very good at it either). If it's PINNACLE then I look closely at ML movements and see what their agenda is. If after looking at everything I conclude that team A wins by exactly 7 appx 20% then I would put appx 1% on +7.5 buying half a point at -120 or less. It depends a LOT on how much I liked team A to cover the 6 though. If I took Team A on Tuesday, and by Saturday I felt that I put too much on them then I may put more on the middle opp as a +EV hedge, depending on the ML I can get on +7 and +7.5. The books hate to get middled in the NFL, and that is one reason the lines dont move much. In the NCAA there are much bigger line moves and I middle a lot more often on those games because there are a lot of games that I dont have much of an opinon on but I know I can get at least 4 points in-between with locals etc. The totals also have some relevance for middling, I like low totals.