Rolling 55, 69, or anything below 5 gives you a critical hit in combat. It is only somewhat dependent on what you choose for the dice setting.I've played with d100 and 5d20 and i always felt the crit chance remained largely the same.
If you are in melee range, then you just roll 1d45. Otherwise if you have the dice setting for 1 die you roll 1d100. Anything else and you roll 2d50 (even if you chose 5 dice).
So that means dice setting doesn't matter for melee or short range attacks. For long range attacks, you have 6% chance of a crit with 1 die and 3.36% chance of a crit with the other settings.