[2022] PSA: Do not run reflect maps with less than 130% reduced reflected damage taken.

But if a player doesn't know these modifiers stack additively (which is described in the in-game help system), they wouldn't think they had 100% (or higher) reduced in the first place, because that didn't come from a single modifier. In the case in the OP, there's a 60% reduced and a 50% reduced, and those can only be seen as combining to 110% reduced if you either know or assume these modifiers stack additively, which at the very least implies the possibility that other things also stack additively with them.

if the question is "you take 100% reduced reflected damage; how much reflected damage do you take?" and the answer isn't "none", that's kind of a trick question

The game never poses that question to the player. In this case it's asking "If you have 50% reduced reflected damage taken and 60% reduced reflected damage taken, how much do you take?", and then "what about if you also have 30% increased damage taken?"

I think puerility meant that it's not obvious that "reduced reflected damage taken" and "increased damage taken" stack additively because they could be in different "categories"

This is true, but neither is obvious that they would not stack additively - I am not arguing at all that the exact result is obvious to a new player, only that it is not unreasonably outside expectations - having already learned or assumed that the two reflected damage modifeirs stack additively, it would be unreasonable to assume nothing else could possibly stack additively with them, so the possibily that something can do so should not be a complete surprise to the player.

Just like "increased damage" and "increased damage taken (on the enemy)" stack multiplicatively (correct me if I'm wrong)

Only because you specifically asked, because this is extremely nitpicky: Those do not stack multiplicatively, they technically do not "stack" at all. In practice, however, their combined effect on the end result result of removing monster life is close to multiplicative stacking in most cases. They are fundamentally modifying two different values, which are usually related but in extreme cases may not have anything to do with one another. "increased damage" is modifying your base & added damage to get your total minimum and maximum damage you will attempt to deal. When you hit something you roll a value between that minimum and maximum, and that value is then affected by all the enemy's mitigation to get a value of damage they take. "increased damage taken" modifies that value. But that might be entirely unrelated to the damage you tried to deal, for example if you try to deal chaos damage to a character with CI but wearing Crown of Thorns - your increase to damage applies to your minimum and maximum chaos damage, then you roll a value in that range, then that gets thrown away but a new 25 physical damage value is created, and the damage taken increase applies to that.

If these modifiers used "less", then the 50% less and the 60% less would combine to an effective total of 80% less (you'd still take 50% of 40% of the damage, for 20%), not 110%, and no amount of stacking such modifiers would remove all of it completely.

It's a horrible pile of lies that technically doesn't actually modify damage, but the damage multiplier applied to damage from being a critical strike, in a way unlike anything else in the game.

What it actually does is look at the enemy's total critical strike multiplier (default 130% for monsters), subtract 100% from that to leave only the part which makes the damage higher (30% for monster with no crit modifiers), apply a percentage reduction to that value (e.g. 10% reduced from this stat would reduce the 30 to 27), then give the original 100% back and treat that as the critical strike multiplier to use for this hit.

The design reason it works that way makes sense - it shouldn't be able to make critical strikes damage you less than non-critical strikes, and 100% reduced extra damage taken from crits should just make them the same as non-crits - but I don't like how it's inconsistent with everything else.

I dearly hope to one day be able to burn it to the ground and replace it with something that isn't such a super special snowflake and which we can describe accurately without extending the description text to a paragraph.

Completely off topic but why is the term 'recently' used to describe different values of time.

It isn't (or if it is somewhere then that's a bug).

I.e. the Warcry Mastery that gives 20% increased Damage for each time you've Warcried Recently is 8 seconds, however almost every other example of 'Recently' I can find is 4 seconds.

That is definitely set to be 4 seconds in the code. Are you seeing this count a longer amount of time in-game?

This is one of several potential changes to that system which have been discussed in the past as options for the long term. Personally I am in favour of something similar to this change, although it would certainly have consequecnes to other things that would need worked out if we ever do it.

For physical damage specifically, we store the pre-mitigation value in the damage package so it can be referenced. The amount of damage "prevented" is the amount by which the actual physical damage taken differs from that pre-mitigation value of physical damage to deal.

So basically anything which lowers the amount of physical damage taken counts as preventing, with the specific exception of "damage taken as" modifiers, which apply before mitigation, and intentionally also before the pre-mitigation value is stored (because you didn't prevent that damage, you just took it as a different type).

And how does Untiring interact with Petrified Blood or Steelskin?

It doesn't, neither of those affect the amount of damage taken - they change what and how much you lose as a result of taking the damage, but they do not affect how much damage you take.

Nah, you're fine, I just dislike the misleading description on that specific stat because it implies it stacks with other things that it really doesn't.

That would mean it did nothing, because there isn't a critical strike damage multiplier applied to the reflected damage for it to modify.

And that wouldn't change any of what's complained about in the rest of this thread because increases to that stat are still additive with reductions to it.

The amount of extra damage can't be negative. In general, reducing things by values greater than 100% in PoE is prevented from negating the value.

That would allow reducing the critical strike multiplier lower than 100%, which is undersirable for this stat, and is more expensive for performance.

"buyer beware" (i.e. your crits will do less damage than non-crits)?

Pretty much this.