Note:
The values used in this test you wouldn't normally see but this was done for testing purposes.

The Crimson Gem has a passive of reflecting damage back at the attacker if you have the Amulet or is attached onto your Armor... however if you have a mod that allows you to upscale the damage taken... it can lead to some... rather high reflected damage.

In this test with Scaling Health since it allows you to upscale the damage taken & increase health... I had a Zombie Pigman attack me while wearing a Unbreakable Protection IV Diamond Armor Set. The Zombie Pigman spawned with it's Health at 2704 while mine was set at 10000. I am also using ToroHUD so I can see the damage particles & mob health.

According to Scaling Health's Debug Info... this is the damage I took from that Zombie Pigman.

[21:23:04] [Server thread/INFO] [Scaling Health]: mob on Zonic_Tonic: 8.0 -> 9608.0 (scale=3.0, affected=400.0, change=9600.0)

I applied Instant Health to keep my health up until I got a proc with the Crimson Gem. The damage the Zombie Pigman took from the Crimson Gem's reflect was 6303 while the damage I took from the attack was 2905, due to the Armor + Toughness + Enchantments.

It seems that there's some form of damage reduction being applied for the reflected hit but it still seems kinda high. Is it possible to change the reflected damage to only deal what was actually lost from absorb/health instead of the original damage? Like for example... original damage was 9608 but due to damage reductions... I only took 2905 damage, which reflects that 2905 damage back to the attacker.

EDIT
Did a test without any armor & damage scaling... seems you reflect half of the damage back at your attacker... I got hit an 8, I reflected 4 back. While wearing the Protection IV Diamond Armor Set... I still reflected back 4 damage despite the damage taken being less than 1.

It appears to be half, but... doing a test again but with the damage scaled but not as much compared to the last test.

[21:38:32] [Server thread/INFO] [Scaling Health]: mob on Zonic_Tonic: 8.0 -> 1928.096 (scale=3.0, affected=80.004, change=1920.096)

...without any armor & such. I took 1928 damage while the reflected damage was 1265.

Hm yeah, I could change it to reflect the effective damage taken. But then is it even worth using?

Might depend. If it nerfs it quite badly... maybe only reflect half of the damage without any reduction?

2905 damage taken, reflects 1425.5 damage. If going with Vanilla Limits... took 60 damage, reflect 30 damage, etc.

The reflected percentage depends on the gem proc strength, which in turn is based on attack damage (balanced around vanilla level attack strength), number of gems and weaknesses between gem types. With your damage scaling mod it ends up always being 2/3rds.

Ah, I didn't know that, actually.