http://wiki.trackbase.net/wiki/Wolfe...nemy_Territory
The rating system is nothing more than a little bit more advanced ELO System, this means to get exactly +0.00 you are expected to do an x amount better than others based on your rating.
In the following example we take the following averages and expected results:
Average rating: 10, your rating: 20
Average ppm: 15, your ppm: 20
You must get at least (your rating / average rating ) * average ppm, in this case that would mean (20/10)*15 = 30 PPM. If you get lower than this average (in this case 20) you will get minus, if you would have got more than 30 ppm you would have gotten a positive rating for the round.