In my opinion the tier avg qualifications are roughly as follows:
Hack=150-100; Amateur=100-80; Pro=80-72; Tour Pro=72-67; Master=67-63; Tour Master=63-60; Legend=under 60
In addition to your average, each tier has its own minimum number of rounds played, with higher tiers requiring more rounds. So it's possible to have an average low enough to advance, but still not do so because the minimum rounds played threshold has not been met. When players advance a tier often their avg will increase slightly because they meet more difficult course conditions (wind, greens, tees) as they progress. But WGT does not allow demotion so once you qualify for a tier you stay there. That's why you'll see masters players with tour pro averages, etc.