As you can see, Codeforces doesn't provide real rating, and we can only use the length of rating changes to calculate the real rating. The official blog wrote that CF rating calculator uses real rating to calculate rating changes, that is, we should get all people's real ratings to predict the rating changes. But method user.rating of Codeforces API can only get ONE account's rating changes in one attempt, so it's quite hard to do it. Hope Codeforces API can add realRating into User object :)