A team which averages 20 yards per game rushing more than it gives up is a team that controls the line of scrimmage. Such a team is usually a strong play at home. Similarly, a team that averages 20 yards or more per game rushing less than its opponent is a liablility, especially when laying a field goal or more. Anything above or below this range is above average or below average. Anything above or below that range is above or below average.
Defense wins football games and usually goes a long way toward covering the spread.
Teams with good defensive ratings are good bets when they are underdogs and teams with bad defensive ratings are good to play against when favored by 3 or more. Yards per pass attempt is a much more telling statistic than yards per pass completion, passing yardage or passing percentage. 18-23 is the range that most NFL teams fall in when they score points or allow them. I would guess that the same team's success in covering the spread isn't far behind. Just remember that the figures you're looking at in the table above represent a per game average.