Some numbers to help me understand this better (you're welcome to double check this, math isn't my strongest point):

For anyone wondering what this is about: there's a discrepancy between what the Shield Blockbufftooltip and what the Shield Blockabilitytooltip is saying (everyone seems to have based their math on the latter apparently). We don't know which one is true, but I'm trying to understand what the differences mean.

OLD MECHANICS(Shield Block critical block chance bonus = avoidance+block-75)

Scenario 1

Let's say our warrior has these stats: 75% avoidance (5% miss, 12.5% dodge, 12.5% parry, 45% block, 30% critical block)

Against 100 attacks of 100 damage, his damage taken is on average

25 * 100 = 2500 [hits]

45 * 100 * (1-0.3*1.3) = 2745 [blocks and crit blocks]

Total dmg taken: 5245

With Shield Block it looks like this:

70 * 100 * (1-0.3*1.3) = 4270 [blocks and crit blocks]

Total dmg taken: 4270

That makes Shield Block equal to an 18.58% damage reduction in this situation.

Scenario 2

Now let's add 10 mastery, and we get: 90% avoidance (5% miss, 12.5% dodge, 12.5% parry, 60% block, 45% crit block chance)

10 * 100 = 1000 [hits]

60 * 100 * (1-0.3*1.45) = 3390 [blocks and crit blocks]

Total dmg taken: 4390

With Shield Block up (assuming it adds 10% block chance and 15% crit block chance):

70 * 100 * (1-0.3*1.6) = 3640 [blocks and crit blocks]

Total dmg taken: 3640

That makes Shield Block equal to a 17.08% damage reduction.

(Shield Block critical block chance bonus = avoidance+block-50)

NEW MECHANICS

Scenario 3

Let's take the same warrior as in Scenario 2, but pretend that the current Shield Block buff tooltip is correct, which means that Shield Block would effectively add 10% block and 40% crit block in this situation.

70 * 100 * (1-0.3*1.85) = 3115 [blocks and crit blocks]

Total dmg taken: 3115

That makes Shield Block equal to a 29% damage reduction.

Edit: added the formulas for each mechanic and a short explanation what this is about.

Edit2: fixed the damage reduction

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