I'm a casual player in a casual guild enjoying tanking before anything else in WoW. Since I'm not too much of an active player, I don't have time to top my gear at the same rate as the other tanks in my guild do. Also, so far my guild is just entering Karazhan and I've contended myself to offtank only (especially since my constant 200~300ms latency doesn't allow me to be very efficient while holding aggro).

Currently I stopped at 515 Def and I'm quite puzzled as to why the other two tanks keep stacking their def. only, up to 553, basically ignoring other specific mitigation ratings (dodge, parry, block). So I made up a calculation to see what's the equivalence between Defence, Dodge, Parry & Block Ratings.

I've found out that for level 70:

14.81Defense Rating (=6,25Defense=1% total mitigation)=18.9 Dodge Rating (1% dodge)=31.54Parry Rating (1% parry)=7.88 Block Rating (1% block)

I'm not particularily interested in the priority between mitigation types (dodge before block or viceversa) but I'm puzzled by the difference in numbers.
I thought that 1 x rating=1 y rating but it seems, by my computations, that it isn't.

Am I doing something wrong? Do I miss something?

Would this make it such that defense is worth more than dodge, as it gives some parry, therefore more threat? (and which is the 20 rating/18.9 rating? If defense is the first, that would make more sense, due to the parry gained.)

20 defense rating gives you 0.33% miss, 0.33% dodge, 0.33% parry, and 0.33% block (and 0.33% crit fortification if you still need that).

18.9 dodge rating gives you 1% dodge.

So yes, there are many ways to compare the two.

This reminds me of a thread I saw in the Warrior forums (that I can no longer find) in which someone tried to create a crit/crush-immune warrior that never used shield block. I could have sworn I saw such a thread, complete with a gear list, yet I could never find it again.

So today for want of anything better to do I was sorting through possible gear choices for maximizing dodge/parry/block and I hit a wall trying to mentally juggle items with defense rating against items with straight-up block (which it seemed easiest to find in abundance, and cheapest in terms of rating per % point). This thread sets me straight on how the various ratings compare, so thank you.

Could be! I idle a lot on this forum and might have got mixed up. Will look at that thread in greater detail.

I don't think that's actually the right thread, although it's along a basically identical line. Perhaps some of the gem socketing choices can be re-evaluated in light of the above ratings comparison. Hrmm. Thanks again. Unless i'm misreading the OP, when shooting for maximum avoidance I should never socket a Dodge (Subtle Living Ruby) gem over a Defense (Thick Dawnstone) gem of the same "tier quality", because they're both +8 of their respective ratings and you get more mileage out of defense?

490 is the number to shoot for to basically be as uncrittable as you can. Past that, there's 2 train of thoughts: one is to stack stamina as much as possible afterward, the other is to add more defense as it adds a little bit of block, parry and dodge.

That said, for off-tanking Karazhan, you would do better and contribute more by going with a hybrid spec or a pure dps build as long as your gear keeps you past 490 defense. There's not much else to off-tank where you need full protection and in many fights, your lower dps will hamper the group.