Does the classic NFL point differential system work?

Before i waste a day or two back testing it, can anyone tell me if it works?

I've been looking at it a bit and think I can tweek a couple of things that suit my liking better to get an idea of the total points scored in the game, but the bottom line i'm asking is, does weighing up a teams relative offence and defence strength based on the teams they've played vs the league averages actually give an indication as to what a games out come will likely be? Or is it a grand waste of time?

Or do i need a little more brain power?