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Update 14 introduced changes in how Armor Class is calculated. This is good news for Monks as you can make more informed tweaks to your protections for better survival.
Prior to Update 14, there was not really a hard-and-fast rule to how much AC you will require, especially at high-levels. With the new calculations of damage mitigated, in addition to other protections, you should have a much better way to see where your armoring falls short as you continue your training.
Your character sheet shows a full breakdown of the various armor bonus types when you hover your mouse pointer over your AC number.
The breakdown is very helpful as you can tweak the bonuses using items, tomes, Monk stances to get better numbers.
Managing each bonus is very important as you increase in training to fight powerful foes, especially in raids and Epic difficulty.
The key to armor bonuses is that most of their numbers do not stack with other items of the same bonus type. There are exceptions specific to Monks that you can leverage for your character.
Let's break down what the primary armor bonus types are available and how they apply to Monks.
Wielders of the Unyielding Sentinel Epic Destiny can take advantage of how Shield AC is calculated.
The White Dragonscale Robes and their improved version, the Flawless White Dragonscale Robes, both have Shield AC bonuses. According to Senior Teacher Regtur (see the comments), the Shield AC counts as a virtual shield. You can train the Shield Prowess ability of that destiny to increase your Shield AC bonus by up to 50%.
All these numbers will calculate a Defense Chance at Level, which gives you a stronger indication of your overall protection based on your training. In this example, Teacher Syncletica (buffed and in Ultimate Mountain Stance) shows a 58% chance to avoid damage at level with 75 AC.
Update 14 broke out Dodge as an AC bonus and redefined it as a percentage chance to avoid damage, up to 25%. This Dodge cap can be increased through Ninja Spy enhancements permanently, augmented by guild ship buffs, untyped Dodge bonuses, or Epic feats and destinies.
Dodge can be just as important, if not more so than AC and PRR, for all Monks. For more on the importance of Evasion, Dodge, Concealment and Incorporeality, see the next chapter.
There are still items that form a Dodge-as-AC bonus, but these items are not designed for pure Monks.
The Physical Resistance Rating also gives a percentage chance to mitigate damage from bludgeoning, slashing and piercing weapons. Even a small amount of PRR may leave you less likely to be injured.
PRR is designed for heavily armored and shielded melee fighters since their weight of armor limits their mobility--specifically, dexterity, and their AC bonus from that stat. As such, Monks will never see the higher PRR as these fighters--or will they?
With Update 17, the addition of a new Augment slot ability has been added to many, many new items that allows the insertion of Augment crystals that can boost PRR (among other things) to very respectable levels, without compromising other effects you may have from named gear (much of which has also been revised with slots). Green, Purple or Blue Augment slots accept the PRR gems that range from +2 to +14 PRR.
PRR stacks with itself in most cases and sources except when defined otherwise, such as enhancement bonuses from augment gems and the Cloak of the Bear. Only one gem with the higher number will be counted.
Any PRR is better than none to reduce damage that gets by your innate Dodge, Concealment, Evasion and/or Incorporeality.
Magical Resistance Rating helps more melee characters to shrug off magical damage. As a Monk, your innate Spell Resistance gained through feats will help more than MRR, but you can gain some MRR through items with the Sheltering property that often adds PRR and MRR bonuses.
Update 29 added Insightful properties to random loot that give stacking bonuses to PRR. In addition, Epic Destiny abilities give stacking PRR bonuses if you have other feats or have trained sufficiently in the destiny. These options can add 100 PRR or more to a Monk. Combine this with miss-chance, AC and fortification and the Monk can survive longer than many other armored classes.