After your promotion, your average resets according to the very first round you play at the next tier. If you score lower, your average will drop... if you score higher you average will rise. That is until you are saturated. Once saturated, your average will only go down.
Just to add on to what Don said, you'll need to play 40 ranked rounds as a Master before your average is saturated.
On an unrelated note, you should consider using your free CC pass whenever you play your first round of the day. Your CC (where you are the sole member) is still at Level 1 after a year. You can easily make it to Level 4 within the next few weeks, and at that point you'll get the Shot Pal for free, which should help you lower your scores.