When I started playing, it was all about high runs, or the number of runs over 3, 4, etc. But I figured out that it was really about the average, and over a long period of games.
I remember when I thought I was about a .5 player, but then realized it was closer to .35 or .4 when I actually tracked it because of missing easy shots that would have not only made that point but also extended my run. I worked on my consistency and speed control and position play and actually went past .5 to the .65 - .7 range when I was playing once or twice a week. I had some bad games at around .4 or so, and some good games at .8, 1.0, etc., but I was more focused on maintaining and slowly increasing that average over time.
It can help to keep track and play both balls, essentially playing a game against yourself, as long as you are diligent about shooting the same shot you would against an opponent - i.e. playing correct speed, safe, etc. Your average will also vary based on the level and type of opponent you play, which is where the winning percentage also comes into play as Jim mentioned.
Scott