The information found on the Fallout Wikia seems to be conflicting.
Under the Damage Threshold article, it states that:
The combined statistics of DT and DR (with DT being applied first) determines....
whereas on the Damage Resistance article:
Damage resistance is applied before damage threshold.
I am confused. Which one is really applied first?
The exact Fallout NV damage formula is as follows:
DAMAGE_TO_TARGET = ( DMG + (CRITDMG x BC) - DT )x SA x HS x AM x DM x PERK_MULTIPLIERS
DMG = Unmodified weapon base damage. DAM is equal to DMG with a weapon at full repair, 100 weapon skill and no damage modifying perks
CRITDMG = Unmodified bonus damage added for critical strike on target
SA = Sneak Attack modifier adds +100% total damage
BC = Better Criticals modifier adds +50% critical damage
HS = Headshot modifier adds +100% total damage
DT = Damage Threshold of target
AM = Ammo type multiplier
DM = Difficulty multiplier. One on normal difficulty, 0.5 on very hard.
PERK_MULTIPLIERS = Other damage multipliers from perks such as Lord of Death, Bloody Mess and Living Anatomy (and all damage resistance perks fall under this category)
As you can see, DT is subtracted directly from weapon damage whereas the resistance is a multiplier after the fact. This means that DT is applied first. (source]
