Sturdy Metal notable only considers base chest armour value

The notable says: "80% increased Armour from Equipped Body Armour"

However, this 'increase' is not treated the same as other 'increases' from Defense mods like Impenetrable and Iron runes.

The formula is: (base + flat) * 1 + (increase%/100) * 1 + (quality%/100)

% increased mods and runes count as 'increases' and apply to the base + flat armour value

Sturdy Metal appears to only apply to the item's base armour and doesn't consider any flat bonuses.

Example: I have an Utzaal Cuirass with 2345 final armour
445 base + 253 flat (698 total)
144% 'increased armour' + 36% 'increased armour, evasion, and ES' from runes
20% quality

(445+253) * 2.8 * 1.2 = 2345 armour

With all my passives, I have about a 3.46 armour multiplier.

When I allocate Sturdy Metal I only gain 1219 armour which is approximate to 445 * 0.8 * 3.46 = 1231

If Sturdy Metal was actually an "increase" like others I'd expect to get ~1932 armour (698 * 0.8 * 3.46)

So I did some more digging and what seems to be happening is SM just adds 80% of your chest's "Item Armour" (the term Polished Iron uses a/k/a the value after all local mods, runes, and quality are applied) and tacks it on to your armour value as the very last step.

That means if you have a chest with 2044 armour and 30% increased Armour from passives/jewels, your armour will be 2657 without SM and 4292 with SM.

0.8 * 2044 = 1635
2044 * 1.3 = 2657 + 1635 = 4292

If you have 80% increased Armour from passives it will still be:
2044 * 1.8 = 3679 + 1635 = 5314

imo this is a pretty weird and unintuitive interaction given how it's worded:
"80% increased Armour from Equipped Body Armour"

If my chest's armour went up by 100, I'd expect that 100 to get multiplied by all my "x% increased armour" passives, just like everything else.

A better wording would be "Adds 80% of Equipped Body Armour's Item Armour to your final Armour"

As a passive, this makes it really bad though with awful diminishing returns. The more armour you have, the less valuable this is because it doesn't scale.

The jewel mod for "#% increased energy shield from equipped focus" appears to work this way, as well, similarly to other effects that are worded ~ "gain additional x."

You gain additional energy shield that is not scaled further by any of your passive multipliers.

E.g., I have a focus with 299 ES (including all local modifiers and quality).
Socketing a jewel that has "50% increased energy shield from equipped focus" adds exactly 149 energy shield. This additional ES is not scaled further at all, which I would agree is not intuitive when it uses nearly the same language as other modifiers affecting your maximum energy shield. The base focus without this jewel contributes about 1900 es to my character's maximum energy shield.


edit: I keep calling foci shields :P

You're right.

All of my modifiers scaling energy shield are of the same type and shouldn't be multiplicative with each other, nor are they. It's effectively 'maximum energy shield contributed by your focus.' I wasn't thinking of it as flat es being added, but my thought process was still separating it from maximum es as if it was.

#% increased maximum energy shield
#% increased energy shield from equipped focus
#% reduced global defences
#% increased global defences