1. Partial resists occur in multiples of 10% (not 25% like pre-WotLK).
2. At a given value of resistance, only 3-4 resist percentages are possible.
3. The long-term average damage resisted at a given value of resistance appears to follow the formula
Average Resist = Resistance/(Constant+ Resistance)
For level 80 mobs, the Constant is 400.
For level 83 bosses, the Constant appears to be 510 (pending final confirmation).
4. The possibility of a given partial resist % is eliminated when the average resistance (from the above formula) is 20% higher. This results in the following thresholds for an 83 boss, assuming the constant of 510 is correct:
6. All damage reduction / amplification effects (Defensive stance, Power of Shadron) affect only the unresisted portion of damage. All effects are still multiplicative, this simply means that when reviewing logs, the resisted portion in parentheses will be a flat percent of the base damage of the spell, with no other modifications (the exception being the reduction of periodic damage from resilience - this effect is applied before the partial resist check).
7. The probability of each partial resist seems to obey a triangular distribution. Mathematically:
C = Constant (400 for level 80, assumed 510 for 83)
R = Resistance
AR = Average Resist = R/(C+R)
x = Possible Resist. This must take values of 0.10*n, where n is an integer.
The probability of any x is thus
P(x) = 0.5 - 2.5*|x - AR|
P(x) = 0.5 - 2.5*|x - R/(C+R)|
Negative probabilities are simply treated as zero. This distribution matches the observed data well.
Graphs and Tables
This graph shows the relationship between resistance and the average amount resisted. These curves follow the formula provided above. As discussed, this average amount resisted determines the probability of any potential partial resist.
The following table lists the resistance amounts (from the above chart) which correspond to each average resist. These amounts are different vs. level 80 and 83 mobs.
This table illustrates the probability of each partial resist at a few common amounts of resistance. These probabilities come from the formula P(x) = 0.5 - 2.5*|x - AR|. The resistance amounts were selected from various combinations of buffs, consumables, and available resistance gear. All values are for a level 83 boss.
The final graph displays the probability of each partial resist, as a function of resistance (for a level 83 boss). This information is the same as that in the previous table, at arbitrary values of resistance between 75 and 765. To read this chart, look on the horizontal axis for the amount of resistance you plan to wear, and trace a vertical line upwards. The curves with non-zero values at this resistance represent the possible partial resists. The magnitude at the intersection represents the probability of this resist.
For instance, at a resistance value of 400, the chart indicates nonzero probabilities for the yellow, purple, red, and light blue curves. These correspond to 60%, 30%, 50%, and 40% partial resists. Visually, the probabilities of each are about 10%, 15%, 35%, and 40%. This can be confirmed with the formula above.