Alternative rating.

http://ratings.fide.com/calculator_rtd.phtml - this seems like a quite authoritative option to me:smile20:
Elo rating http://en.wikipedia.org/wiki/Elo_rating_system
As far as I understand, the current professional Elo system has been used in chess since 1970, in many sports, and esports, and its calculator is provided in the first link.
http://www.namibianchessfederation.com/Pages/calculator.php