your avoidance chance is your parry chance + dodge chance + 5% miss. So if you have 12% dodge and parry you have 29% total avoidance against a level 85 enemy.
To work out the chance for an enemy to hit you instead of being avoided or blocked, you simply add your block chance to this number. Assuming our tank has 51% block, he will have an 80% chance to block, parry, miss or dodge and a level 85 enemy only has a 20% chance to hit him normally.
When you are being attacked by a higher level enemy they can disregard some of your defensive stats. A level+1 enemy ignores 0.2% of each stat per level, so a total of 0.8% pdmb (parry + dodge + miss + block) per level difference. this means that you need a total of 100+2.4 = 102.4% pdmb to no longer take direct hits from a boss level (level+3) enemy. Do not try to reach this, you won't manage until you are in at least T12 gear.
My unbuffed stats in mostly 359 gear are
So if I am not buffed and fighting a boss (it has happened before :/ ) I have (102.4-(12.29+13.39+51.59+5))=20.13% chance for the boss to hit me with an unblocked swing.
Does this help?