Saturation at Legend tier is 500 ranked rounds played at that tier. Par 3s, par 5s, and custom courses do not count toward average. In order to tier up, a player needs to have those 500 rounds and an average of 60.00 or lower.
For those who follow the traditional route (i.e. ranked rounds) levels don't matter. Realize, however, that WGT has different criteria for coin players that (in part) uses levels as one of the criteria.
Once a player tiers up, his or her average resets with his or her first ranked round played at the new tier. So, a player could tier up to TL, then play a ranked round of 76, and his new average would be 76, even though he is a TL.
Averages are calculated only on the ranked rounds within the tier the player currently is at.
I hope that helps.