every tier has a certain number of Ranked Stroke play rounds that you need to play before the average saturates and only goes down.
H- 5
A - 10
P - 20
TP - 30
M - 40
TM - 50
L - 500
TL - 500
something like that, so once you reach those rounds in a given tier it will then only go down until you tier up, then like Opy said, start from scratch.
Don't worry so much about the average anyhow, because of the saturation and the easy way to manipulate the average prior to saturation. It really doesn't mean a whole lot. You can have mediocre players with sub 60 averages, just cuz they played a given unlimited tourney over and over again until they posted the score they wanted, quitting or restarting before posting the score. You can have extremely good players score badly in ranked rounds to make it look like they suck but they don't.
Just play and have fun and work on improving your game