[sandalman]

almost.

u have 21 rated rounds;
calculate the average of 21 rated rounds;
calculate the SD from the 21 rated rounds;
calculate your range - multiply SD by 2.5 (use 100 if result > 100);
subract the range from the average to get your lower boundary;
remove any rounds rated below your average;
divide 21 by 4 ( = 5);
weight the last 5 rounds double;
your rating is the average of the rounds with doubling