+% armor/res skills and gear choices: a pure math approach
I don't know if anyone's interested, but if you wondering how things like War Cry effect your gear choices, I wrote this document which goes over the relevant math and two sample equations that model how one makes gear choices.
The long and short of it:
Having a +% Armor skill, like the +40% War Cry, does not entitle you to have a lopsided, high-Armor build at the cost of resists.
Having a +% Armor skill does makes Armor slightly more attractive than it otherwise would be, but the effect is smaller than you might think.
At no point does any skill make the +Armor mod better defensively than +res all.
Re: +% armor/res skills and gear choices: a pure math approach
Originally Posted by zakaluka
How much would I have to bribe you to clone the process for steel nerves and iron impact?
Nerves of Steel isn't a +% armor skill; it just adds Vitality to it. Tough as Nails does too; the gem economy difference there is 10*level, so at enemy level 63 you want a 630 armor/63 res difference. No contest in affix economy. A side note on Tough as Nails: It's value comparative to Superstition (vs non-physical damage) is equal to your armor mitigation percent BEFORE equipping Tough as Nails; for example, if your armor mitigation is 63%, Tough as Nails is 63% as effective as Superstition. Since armor mitigation will never reach 100%, Superstition will always be better (except, of course, against physical).
Iron Impact has a cooldown of 10 seconds and an active buff time of 4 seconds. This makes it both hard to depend on in practice and a real computational pain in theory. Let's go over the possibilities:
Completely on. This would assume you have amazing micro skills and always control the situation so that you never take hits except when Iron Impact is active. In this case the gem economy is easy to calculate: A = 10R + 75/2*L. That makes for a big difference: at level 63, a 2362 point difference! Amazingly, this skill STILL failed to penetrate the affix economy. Resist all is powerful indeed; the bottom two situations will not be calculated for affix economy (which is turning to be an impossible standard).
Completely off. This assumes you get no benefit at all; ALL relevant damage occurs while this skill is locked away in cooldown. This wouldn't necessarily be due to play error. For example, you are slogging it out against this mob that you easily tanked with Iron Impact on — so easily that you leeched life back up to full. However, once the Iron Impact duration ends, you start to take damage fast — so fast that you can't survive to 6 seconds to get back to a point that you can leech back up again. Also easy to compute: A = 10R.
Proportional on/off. This assumes 60% of the hits are taken during cooldown while 40% receive the Iron Impact benefit. This was devilishly difficult to compute; it didn't even end linear. The equations (and a chart) are here while a spreadsheet with a table is here.
Conclusion on Iron Impact: I think the proportional on/off tells the story best, with a lean towards completely off. Iron Impact on a pure armor build seems pretty useful when first unlocked; like Tough as Nails, the effect on EHP is proportional to original armor mitigation, and early on that means Iron Impact represents a temporary x3-ish multiplier to EHP. However, the real worry isn't while Iron Impact is active, but when it's in cooldown. Characters should hope of the best but prepare for the worst.