Cast when damage taken gem - noob question?

Hey, i got a noob question belonging "cast when damage taken":
Do i actually have to receive the damage to my for example lifepool in order to trigger it?
Storm secret lets me take 250 lightning damage, 2 = 500 when my herald of thunder strikes an enemy.
If i were to mitigate the damage by lets say 50%.
Do i take 250 damage for the "Cast when damage taken gem" to proc? Or the "original" 500?
I hope the question makes sense. I cant find a definite answer to whether the gem counts the "actual damage taken to the lifepool" or the damage taken before mitigation or taken as etc.

When i have -200% lightning resistance and no "taken as" or other mitigation, i'd take 2000 lightning damage with 2 stom secret rings. I could then level my cast when damage taken higher. But would need lots of life recovery, or recoup, or whatever to survive. Because "damage taken as" would shift around and doesnt actually hurt my lifepool when i was about to mitigate the dmg with resistance or whatever...

Would it (the cast when damage taken gem) still count 2000 damage, if i shift lightning damage to fire and mitigate it afterwards?

Thanks in advance

Afaik taken means after mitigation and taxes.

Yea thats what i think as well.
Thank you!
Hoped it would be otherwise

Damage taken refers to the amount of damage taken after all mitigations have taken place.

https://www.poewiki.net/wiki/Damage_taken#:~:text=Damage%20taken%20refers%20to%20the%20amount%20of%20damage,of%20%27damage%20dealt%27%2C%20but%20from%20a%20defending%20standpoint.