Source?
Crit Chance = Base Chance (Luck + any perks, etc) x Crit Multiplier (this is dependant on the weapon being used) x Weapon Condition. If VATS is being used then the final number has +15 added on.
Skills, agility and hit location arnt included in crit calculations in the slightest.
I don't have a source I'm just basing it on common RPG logic.
Edit: Actually a quick Wiki skeg: http://fallout.wikia.com/wiki/Critical_Hit shows that I am wrong. "Note that the chance for a critical hit is determined by Critical Chance, and is not the same as critical effect." I guess I'm talking about critical effect, then, the damage done when the Critical Hit has been decided after passing the critical chance.
I bow to your knowledge sir.

I'm running on like three hours sleep over a seventy-odd hour period, I shouldn't be allowed to post. lol :laugh: