By Leviathan.Vow 2013-08-23 17:36:32
It's possible I'm oversimplifying, but if you look at what DEF and PDT do you can come up with a quick approximation.

PDT reduces the amount of physical damage you take by a direct percentage. If you take some amount of initial damage,

Damage Taken = Initial_Damage*(1-PDT/100).

Assuming that the monster's damage is calculated in the same way that a player's damage is, defense reduces the amount of damage you take in a slightly different way. From BGwiki:

Physical Damage = Base Damage * pDIF

where Physical Damage represents the monster's damage. Defense affects the pDIF term, which is a ratio between the attacker's attack and the target's defense.

Assume the monster has some amount of attack, Monster_Attack, and that you have some amount of defense, Player_DEF. The monster's pDIF will be calculated as

Monster_Attack/Player_DEF,

and then it's damage will be calculated by multiplying by base damage. If you increase your defense to some value, Player_DEF', then the monster's pDIF will be reduced to

Monster_Attack/Player_DEF'.

The ratio of the damage you take with your new defense value vs. the lower defense value will then be given by

Physical Damage'/Physical damage.

The Base Damage term drops out, as nothing has changed, leaving only

pDIF'/pDIF,

that is,

(Monster_Attack/Player_DEF')/(Monster_Attack/Player_DEF)

which reduces to

Player_DEF/Player_DEF'.

All you have to do to compare the amount of damage you'll take by increasing defense is to divide your old defense by your new defense.

If you want to compare PDT to DEF for a particular piece, you can directly compare the two:

Player_DEF/Player_DEF' = (1-PDT/100).

A quick rearrangement of terms gives

Player_DEF'=(Player_DEF)/(1-PDT/100) which will tell you how much defense you need in order to gain the benefit of some amount of PDT given some initial amount of defense.

This doesn't work if the monster lands a critical hit, though. Critical hits add 1 to your attack vs. defense ration, so the attack term doesn't drop out. That is, you end up comparing

(1/DEF' + 1/ATT)/(1/DEF + 1/ATT).

That's about as good as you can do with a quick approximation, but pDIF includes one more step which I've ignored.