This guide will explain how the game calculates the amount of damage that is absorbed by the various types of armor, using an example. The percentage values wouldn't be added together, as that would actually give the player Health back when they were hit. Therefore, a 'Decision Tree' must be calculated. It should be noted that the order the armor is equipped will make a difference in which armor will lose the most Durability first. Whatever item is equipped last will be the item to take the most significant chunk of the damage as it is utilized first in the damage calculation (see the scenario at the bottom for a sample situation).
Here's an example we can use.
In your helmet slot you have a Football Helmet, which will absorb 80% of all damage dealt.
When put together you receive 92% of total damage absorption, NOT 140%, which would actually add health points to your total. Now, as cool an exploit as that would be, it really doesn't make for an excellent game.
dmg(taken) = dmg(dealt)*(1-[last equipped absorption])*(1-[ first equipped absorption])
Scenario: Assume the football helmet is equipped first, and the grass suit was equipped second. An enemy attacks once and the total damage dealt was 100. Since the grass suit was equipped last, it will take the damage first. The grass suit has a 60% absorption rate and 225HP. The strike will reduce the grass suit by 60HP(leaving it with approximately 73% durability. The Football Helmet takes damage second since it was equipped first. It has an 80% absorption rate, and 450HP - the strike will reduce the helmet by 32HP(leaving it with approximately 93% durability). Finally, the player absorbs the remaining damge that the armor didn't reduce meaning the player will lose a total of 8HP from the strike.
Reverse Scenario: Now lets pretend the grass suit was equipped first, and the football helmet last. The same attack happens as above, but this time the football helmet takes the strike damage first. The helmet will absorb 80HP of damage this time, leaving it with 370HP or approximately 82% durability. The grass suit will then absorb 12HP of the 20 leftover, reducing it to 205HP or approximately 91% durability. The player will still absorb the same 8 HP as the first scenario, but as can be seen here, the durability of the equipment is the major changing factor.
It becomes obvious that it's rather important to keep in mind the order you are equipping your armors and helmets. It seems the best strategy here is to equip the hardest to acquire armor/helmet first. What raw materials are required for each and how renewable are they? Comparing the materials for a grass suit vs. that of the football helmet, one would be wise to equip the grass suit last in order to preserve the football helmet as long as possible. Its far easier to come up with the grass and twigs in most cases than it is to acquire the pigskins needed for the football helmet... with one caveat; the grass suit having a weaker absorption rate and half the HP means it will break much quicker when it is equipped last - so you will want to have backups on hand in order to have consistent protection. If the grass suit breaks then the football helmet will lose durability much quicker anyway, negating the benefit of having it equipped first.