Is there a percentage my block will be at that lets me know I have enough mastery? Does the actual mastery rating stat let me know this?
In general you should stack mastery until you hit the avoidance cap. Bosses get a bonus to hit, so total avoidance need to be 102.4%. Then the boss has a straight 5% miss chance so you want dodge+parry+block to get as close to 97.4% as possible. Once you reach that combined value, start switching from mastery to dodge and parry fairly equally (they suffer from diminishing returns so stacking one or the other is less valuable that spreading equally). The reason to try to cap avoidance with block initially is that even though it doesn't fully avoid the hit, it helps smooth damage spikes significantly, which is a big help to your healers.
The avoidance cap for bosses is 97.4% combined avoidance (+5% chance for the boss to miss), so up mastery, dodge and parry until you hit that. Typically, dodge and parry are preferable to block, but you'll get more chance to block per point of Mastery than chance to dodge or parry per point of those, so stacking mastery is easier early on. Once you can get to the cap with stats left over, you should start to reforge the surplus mastery into dodge or parry instead.
Mitigation is nice, but pointless unless you can keep aggro. If you have threat problems, you may want to temper your focus on mastery/block/dodge/parry in lieu of more expertise and hit (which should be capped in the first place).