I searched F3 and F:NV and can't find a single thread even remotely related -- but I'm sure some of us are curious about critical dmg!

1. How is critical chance calculated? Which do we multiply, and which do we add?
2. What is the multiplier for scoring a critical hit? Is it just x2 dmg?

e.g. my character with his Unarmed/Hand-to-Hand weapon Bladed Gauntlet (which has +crit chance, +crit dmg):

Built to Destroy perk: +3% to crit
Finesse perk: adds 5 points to luck for purposes of crit calculation i.e. adds 5 to base crti chance (from strat guide)
Bladed Gauntlet weapon: "crit % Mult" is 2 (from strat guide) -- What does this mean?
Better Criticals: 50% more dmg when critting
Luck: 6

So I'm guessing that when I hit normally, my damage is:
25 "Bladed Gauntlet base dmg" = 25

Meanwhile, when I score a critical hit, I guess that my damage is:
40 "Bladed Gauntlet base dmg (when critting)" x 1.5 "Better Criticals" x 2 "Critical Multiplier" = 120

Finally, I guess that my chance to crit is
6% "Luck" + 3% "Built to Destroy" + 5% "Finesse" ] x2 "Bladed Gauntlet" == 28%

Please, anyone care to add? Thanks!

The crit % mult of the weapon has nothing to do with damage, it only has to do with your chance to crit. V.A.T.S. gives an additional 15% chance to crit unless things changed since Fallout 3.

I would presume that it's the same system as Fallout 3, in which case, is explained here:

http://fallout.wikia.com/wiki/Critical_Hit#Fallout_3

! I'm glad that F3 onwards removed the obtuse crit effect table from F2 and before. Since a lot of perks and game mechanics get refined/nerfed through each fallout, I'm not holding my breath that F3 and F:NV are identical, but I'm guessing that they'll be really close (both have VATS system). Thanks for the link!

Critical Strike Chance = Base Crit * Weapon Multiplier * Weapon Condition. You are right that you have a 28% chance to crit with the bladed gauntlet but only if its at peak condition. At 50% condition you only have a 14% chance to crit. When you score a critical hit it adds a certain fixed amount of damage (which is increased with the better criticals perk).

An example-

Weapon X has:
20 base damage
5x crit multiplier
30 crit damage
25% condition

Character X has:
6 Luck (6%)
Built to Destroy (3%)

The chance of a critical strike is calculated by the formula above. So base crit is 9% (6 Luck and BTD), this is multiplied by 5x because of the crit multiplier bringing us up to 45%. The weapon however is in poor condition, this brings it down to 11.25%

When a critical hit is scored the weapon deals 30 bonus damage. So in this case we have an 11.25% chance to deal 50 damage on a critical strike. If we had the better criticals perk then the 30 crit damage would be increased to 45. Meaning we would deal 65 damage on a critical strike instead.