Tier progress in 'My profile'
It's a moving target, based on your scoring average and number of ranked rounds.
Each tier requires a minimum number of ranked rounds be played, and when that is satisfied you 'tier up' when your scoring average reaches the next 'tier up' level.
I know I would 'tier up' again when my scoring average reached 61.00, which it did a couple of days ago.
Before you reach the minimum ranked rounds requirement, your scoring average can go up or down. With further back tees, and trying some different make/model balls, my scoring average has gone up to 66.11 because in the new tier I haven't yet come close to meeting the minimum ranked rounds requirement yet.
After you reach the minimum ranked rounds requirement, your scoring average can only go down - If you shoot a high score, your scoring average doesn't change.