The table at the bottom of this page relates expected game results to rating differences. The P column is the expected Percentage for the result of a single game. The D column is the rating Difference corresponding to that expected result.
For example, two players with the same rating (D=0) each have a 50% (P=.50) chance of winning a game. Similarly, a player with a rating 100 points greater than an opponent (D=102 is the closest value in the table) has a 64% (P=.64) chance of winning a game.
Let's say you score +3-2=1 (three wins, two losses and a draw) against opposition with an average rating of 1500. Your score is 3.5-2.5, for a percentage of 58% (P=0.58). The value for P=0.58 in the table corresponds to D=57. Your performance is calculated as 1500 + 57 = 1557. If you had achieved the same score against opponents with an average rating of 2000, your performance would be 2057.
This method is used to calculate an initial rating for a previously unrated player. The more games used in the calculation, the more accurate the initial rating will be.
