Mortal Blow (and more)

Ok, so I have a Mortal Blow (MB) question. I'm currently a 25 Assassin, fighting zombie soldiers in the ruins south of gludio. They give nice xp, and don't aggro so I can get behind them (thats part of my question, btw).

So I usually start the fight lining up behind the zombie and doing MB. I'm trying to figure what affects MB.

MB only lands on a critical, right? I mean, in the log it says,
"Use Mortal Blow
Critical hit!
blah blah blah damage"
So thats why I assume it only lands on a crit. So what affects crit rate? DEX? Accuracy? Being behind the mob? If accuracy affects crit rate then it would be good to line up behind the mob, hit the accuracy skill (the one that drains MP while in use, but improves accuracy), then hit MB. If its DEX then it really doesn't matter, can't really do much about that, until tats.

Also, are sting and bleed any good?

Your dex stat plus and focus type buffs affect it, but not Accuracy as far as I know.

Oh, and Chonicle 3 had more affect on dagger skills than any stat or buff ever could. :p

Well, nothing affects landing rate of MB/Deadly blow...
But positioning, mighty mortal and luck.