It's not hard to determine from a box score how many points your team scored, but it is difficult to determine how many possessions you had. The ball can only change hands four different ways: (1) your team makes a shot; (2) your team misses a shot and doesn't get an offensive rebound; (3) your team turns the ball over; and (4) your team goes to the line for two or three shots and either makes the last shot or doesn't get the rebound of a missed last shot.
The first three of these things can easily be calculated from a box score, but the last one cannot - just by looking at the amount of free throws taken, you can't tell how many of these were possession-ending free throws. However, our analysis of NBA free throw patterns shows that approximately 43.6% of all free throws are possession-ending, a result that makes sense when you consider the number of "and-1" and technical free throws, when the shooting team doesn't lose the ball.
So if you're willing to use this estimator on your free throw totals, the total points per possession (or, more accurately, points per 100 possessions) of any offense should be:
PPP = (100 x points scored) / ((FGA - O-Rebs) + TO + (.436 x FTA))
Of course, this won't be exactly accurate over small periods of time, simply because the number of possession-ending free throws isn't exactly 43.6% of the total all the time. But it should be reasonably close to the actual total if you're including at least several games' worth of data. Got it?