Scarlet Delirium i tested it a bit more today and here's the finding.
On use Effect that will last 1 min
Once you take any DMG you will gain the Scarlet Delirium which will last 1 min after that. (aka if you take DMG 30secs in you will get the Scarlet Delirium effect for 1 min after that 30 secs)
Any dmg after will add no extra DMG. ( mean if a mob hits you for 100 DMG ur looking at a 1% DMG boost for the entire effect)
It seems to be based off hp % lossed at 1% hp lossed = +0.4% DMG (very rough estimate)
All the testing info is on the DRK forum and it was done with elemental WSes but it follow what the Dev posted a while back.
I would like SE to answer how they think DRK could use this

u need to take a decent amount of DMG in one hit for it to be worthwhile.
So i guess when i said in best case your getting a 6% DMG overall i was wrong in most cases you'll prolly be looking at a 1% DMG boost.
My last suggestion on this....
If you want to keep it the same way SE make it so
SD Reduces DMG taken 50-100% for the 1 attack and you transfer the DMG to DMG%
Aka SD absorbs DMG taken for a DMG boost. (Work well with the Way drk is absorbing and such)
So you could use like this
Catus rdys 1000 Needles
DRK uses Scarlet Delirium
Catus uses 1000 Needles
DRK takes 0 DMG (and gain 10-20% DMG boost for 1 min)
This way the ability may not be great for dealing DMG but in cases where mobs use highly DMGing move DRK can use that agianst them and this i feel is what DRK is about ^^.