For example, if you had 4000 mitigation and then found an item that had the stat +1% mitigation then you'd see about a 0.75% increase in your survivability.

To work out how much mit your persona window should show you can use this equation:



Where:

G is mit purely on your gear (when naked ie completely unmodified).
M is your total +mit% amount. Entered as a whole number, so if you hve 1.4% on an item you enter 1.4 here.
B is your Defensive Stance worn armour effectiveness bonus (15% for most tanks). So for them it would be 0.15. My Guard has a multiplier of 0.2.
N is the number of items of armour you are wearing, yes, the def stance bonus becomes smaller if you're not wearing all your armour.
Z is a logic point. Make it 1 if you're in Defensive stance, make it 0 if you're not.
P is mitigation from any and all spells/buffs and Personal Advancement choices (you can choose Physical Mitigation instead of Elemental, Noxious or Arcane) Yes, benefit from these is also dictated by number of bits of armour worn.

So far as I'm aware that is ALL of the things that can modify Persona Mitigation.

If you want to turn that number into a Percentage then:



Capped at 75% (0.75)

Where Pmit is the earlier calculated PersonaMitigation value and Y is a correction factor to account for differences in the curve at lower/higher mitigation numbers:




So for Pmit values less than 1720 you want to use the top line for Y, if it is more than 1720 then you want to use the bottom line for Y.