A few main questions should be answered:
- Does Primal have Wrath resistance mechanics, pre-Wrath mechanics (unlikely), or neither?
- What are the resistance caps?
- How much damage resistance do varying amounts of resistance provide against different level NPCs?
Basic Resistance Mechanics
First, there have been various discussions about resistance "caps." Many people have said various things about caps that I cannot help but suspect are incorrect. In vanilla and TBC, there were various soft caps that could be arrived at to achieve a certain level of resistance. Many people say that 365 is "the cap," but while this was the hard cap at 75%, it was also the highest of multiple (soft) caps.
In Wrath, however, resistance was changed. These soft caps were removed, and the the resistance system provided resistance in increments of 10%--every amount of resistance would result in resisting non-binary spell damage by, on average, the same % of binary spells that would be resisted. However, resisted damage wouldn't be by a certain %--it would provide chances of resistance, with those chances distributed across the four closest sections of 10% resistance (eg. 30%/40%/50%/60%). So, in this sense, while there would not be caps, a tank looking to avoid ever resisting below 30% damage would have to achieve a certain level of resistance (where the average resistance is 20% higher than that resistance--in this case, 50% average resistance to eliminate the 20% box):
Here is a chart for some average resistance levels. You can skip the equations and just know that the important number is the resistance required for the average resistance of 50%.
Specifically, the equation for average resistance is:
Damage reduction percentage = 100% * R / ( K + R )
R is effective resistance rating, K is a constant.
To eliminate the chances that a resistance % will be above a certain level:
P(x) = 50% - 250%*|x - AR|
P(x) is the probability of an x% resistance and must be 0 or above. AR is the average resistance from the first equation, or R/(K+R).
Resistance vs. 73 Elite Bosses
Second, the question is: what is the constant for a level 70 character against a level 73 elite? For a level 83 elite, it was believed to be 510 (although some contested this and thought it could be a bit lower). This constant was also the % required for an average of 50% resistance. Does this mean that the additional 3 levels from a level 80 character to a level 83 elite will result in the same constant for a level 70 character against a level 73 elite? This may seem to be the case, as Wrath resistance discussions found that resistance went down 2% for every level* below the caster. But afaik this constant was estimated after a lot of testing with level 80 characters. Some calculations in the EJ thread pointed out in posts below show that the constant for 73 elite bosses is 460 (the resistance required for an average resistance of 50%, and no resists lower than 40%).
One the one hand, if resistance mechanics are Wrath-like this isn't necessarily the most important question (the important thing being balancing HP/avoidance/threat with resistance). On the other hand, figuring out a cap to totally avoid the very low likelihood of a string of say 10% reductions may also be deemed important--while the numbers may not be exact, safely passing that cap if there is a good tradeoff for resistance from other stats may make this valuable.
Resistance vs. 71 Elites
Third, many resistance tanks right now will be tanking Hydross spawns, which should be Level 71 elites. These will have their own constant. If resistance is based on level difference, the constant K will be between 350 and 460 (likely between 375 and 436). The EJ thread featured various models for the constant, but I'm not sure if people settled on any one of them. One person suggesting multiple models seemed to think around 375-400 would do the trick. To give a gist of what different resistances would look like: For tanks with double FrR/NR resist sets and assuming K=435, getting 30% average resistance and supposedly hitting the "cap" to knock out 10% resists requires 187 resistance. The 20% average resistance, knocking out chance of zero resistance, would be 109 resistance. These numbers are pretty speculative but should at least give you an estimate of how much resistance to expect regarding Hydross adds.
The question of whether this is like Wrath or not wasn't addressed. I didn't see any guides regarding resistance yet here, although perhaps people in "vanilla" content who did Rag would know. But it's a possibility. For example, some peoples' perspective is that boss resistance is totally off and bosses are resisting far too much spell damage. I have noticed a lot of resistance on bosses; if that is incorrect, and if resistance issues against bosses could also apply against players, it could be different here.
*If resistance per level is a 2% difference, and a racial passive ability is 2% chance to avoid any damage from a certain school, does this 2% racial affect the calculation, or is it separate? I'm assuming the latter (admittedly a total guess, since the constant is based on 50% resistance and the racial is based on 100% resistance).