As JS said, there are up to 500 rounds in the calculation, and as long as it's not saturated, changes apply in relation to your current average which equals to -8.7 relative to Par 72.
So, if you take your best score of 27, doubled to 54, the difference to your current ave. is 9.33. Divide by an assumed no. of rounds of 400 => 0.023 movement of your average.
As a table for 400 Legend rounds:
score 27 => -.023 movement
score 29 => -.013 movement
score 31 => -.003 movement which is invisible most of the time.
It's the math of big numbers...