These are the Item I tested.
Broken Anti-Dark Property Bracelet (Buff said 10% test result 10%)
Slightly Broken Anti-Dark Property Necklace (Buff said 25% test result 20%)
Rhevisan Bracelet (Buff said 20% test result 15%)
For side note it reduce final damage.
And if you equip 2 pieces it’s calculate multiple time
like 20%+10% ----> 0.8*0.9=0.72
This is my test. Not sure if i do something wrong.
Skill use Dark Theurge lv 5 base damage 1219
Attribute DT 40 attribute Warlock 10
Need someone to check with me. Thanks.