Quote from PiousFlea
Figuring out Defense, Armor and Resistance scaling should be very easy if the mechanics are similar to WoW.
The formula from WoW is:
%DR = 100% - (100% / (x*Armor + 100%)), where x is a different number for each character level.
On the other hand, it appears that Precision is directly proportional to Crit, as in:
%Crit = y*Precision, where y is a different number for each character level
Since both of these formulae are directly proportional (ie, linear no threshold), you technically only need one datapoint to figure out the scaling value - simply divide one value by the other. However, to account for rounding error you will want to average out at least 3 datapoints.
If %DR is the displayed damage reduction (from Defense, Armor, or Resistance), then let %ieHP equal incremental effective HP (ie, the percent gain in effective HP over zero damage reduction):
%ieHP = (100% * %DR) / (100% - %DR)
This can be shown to be directly proportional to Armor as below:
%DR = 100% - (100% / (x*Armor + 100%)) -%DR = -100% + (100% / (x*Armor + 100%)) 100 - %DR = 100% / (x*Armor + 100%) 1 / (100% - %DR) = (x*Armor + 100%) / 100% 100% / (100% - %DR) = x*Armor + 100% (100% / (100% - %DR)) - 100% = x*Armor (100% * %DR) / (100% - %DR) = x*Armor
However, if there is a threshold for Defense (ie, below a certain amount of Defense you get zero damage reduction) the formula will no longer be direct proportional. This appears likely as I've seen several screenshots of players with positive amounts of Defense but 0% damage reduction.
Another thing we are unsure of is if there are any hard caps or diminishing returns thresholds. I suspect that kind of info will only become available once people are at level cap though >_>.