It's bad that I noticed this thread only today. So let me write the correct version for "Critical hits explained".
In Fallout 2:
First, the chance for a critical hit. It's percentage value is a sum of Bonus Critical Chance (gained from Luck, Finesse and More Criticals perks and aimed shot) and a base chance, that depends on the main attack roll. It is equal to the difference of your tohit chance, and the actual d100 number rolled when determining whether the attack was a hit or a miss, divided by ten, rounded down.
So, for example, if you're shooting an opponent with 95% hit chance, and you rolled 64 (which means a hit), then your base chance is (95-64)/10 = 3.1 [%], rounded to 3%.
Final critical chance for a character with 6 Luck and no relevant perks and traits aiming for enemy head would be therefore (3 + 6 + 40)%.
That's right - the main attack roll is used not only for determining the hit/miss, but counts for a critical hit (or failure) chance too.
Now assume that the attack was a critical hit. Follow the
http://falloutmods.wikia.com/wiki/Critical_hit_tables . Multiplier is applied to the base weapon damage, which is increased by the Bonus Ranged or Bonus Melee damage, but NOT increased by the Living Anatomy. This is also done simultaneously with the ammo damage modifiers.
In Fonline:
There are theree main differences: first is the reworked critical hit table (still WIP).
Second, there is a separate critical hit table for an uncalled shot (they were present in FO2, but unused; uncalled shots were identical to torso shots).
Third, the critical chance bonus from aimed shot is reduced to (50 + Luck*5)% of the original value (Luck 6 and eyes gives (50+6*5)% of 60%, which is equal to 48%).
Also,
http://fonlinedocs.netii.net/fo2calg.pdf - explains the FO2 critical chance in a more ordered fashion, as well as the critical failures.