I'm mostly writing this blog post because I actually did some napkin math for a post to the Blizz hunter forum just now and I'm so proud of actually having done something - even though it's something that's already been done by plenty of other people - that I wanted to post it. The root cause of the post is the fact that people aren't sure if they should be stacking mastery as a Survival hunter. There's more to the discussion of crit vs. haste vs. mastery, but the relationship between crit and mastery is pretty much greek for a lot of people.
The first thing to understand is that critical strike chance increases the value of Survival's mastery. Why is this so? Because mastery gives all of your magical damage a percentage bonus of extra damage. Critical strikes make your damage bigger. A ten percent bonus to a bigger number is more valuable than a ten percent bonus to a smaller number. The more often you crit, the more often you make those big numbers that your mastery can affect, the more benefit you're gaining from your mastery.
The second thing to understand is that critical strike chance decreases in value as a percentage of your DPS the more of it that you have. To see why this is, we'll have to do some napkin math.
Ok, so let's say you attack once per second for 1,000 damage per attack. Clearly, you're doing 1,000 DPS. Let's use a test case of 100 seconds.
We'll say that criticals do 50% bonus damage, which is to say that a critical strike does 1,500 damage. If you go from 0% crit to 1% crit, one of your 100 shots will now do 1,500 damage. This will mean that over the course of 100 seconds, you will do 100,500 damage instead of 100,000. This means that 1% crit has brought you from 1,000 DPS to 1,005 DPS.
Let's make it two percent crit. That brings your total damage done to 101,000 or 1010 DPS. Three percent crit chance makes for 1015 DPS. So every time we add a single percent critical strike chance, we're gaining a total of five DPS.
But check this out: that first percentage point of crit chance we added improved our DPS by .5%. That is: 1,005 DPS is .5% better than 1,000 DPS. The next percentage point of crit improved our DPS by .4975%. That same 5 DPS improvement has reduced in value as a percentage of our DPS when we add another percent chance because it's being added to a bigger number. By the time we get to 50% critical strike chance, or 1250 DPS, another 5 DPS only improves our overall DPS by .4%.
Again: The more critical strike chance you have, the less percentage increase you see from the next point.
So if you've been stacking crit and stacking crit and stacking crit, you will eventually reach a point where, yes, mastery overtakes critical strike chance. Optimal DPS will be arrived at maintaining a ratio of critical strike chance to mastery. I'm under the impression that the mathy people like Zeherah are thinking that's a ratio of 4:1 or 5:1, but don't quote me on that.
The thing to keep in mind with that ratio is also that raid buffs count, so it may be that in a raiding situation you can reach a 5:1 ratio even in T11 gear, I have no idea. So if you REALLY want to get crazy about maxing your DPS, you'd have to figure out what your average critical strike chance in a raid is and then get mastery to a point where it's about 1/4 or a 1/5 of that number.
I'm definitely not that crazy. Nor do I play main-spec Survival. It was still interesting to work through the reasoning for myself, though.