Mr. J et. al, a general rule of thumb in the NFL is a 1/2 point is worth another 10% (or at least that is what you are charged to BUY the "hook" (a half point).)

Like Mr. J said, Key numbers are where you want to buy the half point (3, 7, 10, and even 14) Not that it can't come into play but the law of averages are in your favor with key numbers.

There is also a chart for betting the money line (Divvie) and it's comparison to the point spread. For example, -140 = -3 POINTS or +120 = +2- pts. Not exact but a close example. Actually this applies for college football as well. This is the real football to make money on as the information you have can give you quite an edge on soft lines and lazy bookies.