One other thing to remember is that mastery has an active component, mainly from
enrage - critical strikes with special abilites and critical blocks enrage you, increasing rage generation by 25%
This makes mastery a resource generating ability on top of mitigating damage. Avoidance may be better at reducing damage passively but mastery should scale better with player skill. Since shield block also produces a finite amount of guarantees block we're going to have to assume a default boss swing timer to compare avoidance with mastery as well as figuring out how much crit block mastery gives.