New + / - Metric To Measure Impact on Winning

I frequently manually calculate my + / - score for games. This helps me understand how well I played overall.

As a fan of other sports, I think it is more intuitive than the Overall Performance Rating.

My definition of + / - is:

1) Did I cause us to win or lose the point, by either hitting the final shot or hitting the set up shot.

2) Did someone else just hit a great shot with no fault by me?

3) Was it just an unforced error by someone else?