It says it does double critical damage. Whats is the critical hit damage based off of?

I'm trying to make a two handed build though its hard since i'm spreading my perks into block to allow myself a greater variety of combat.

The weapon type specific perks, Im guessing that they are much worse than they sound? I can't remember its been a while.

I just thought i would post these questions here so there can also be a 2hander discussion.