Maxed Wyrd Sorc vs Endgame

when Vanarchy use 10000tg on Psionic Storm build,he can't do it fast clean T10,except 100%demioss

tooltip said 50% of DTU/D but when i add 40% DTU/D by

Relic (Widowmaker)

Relic
Required Level: 75
+(21 to 40)% Damage to Undead
+(15 to 24) to Widowmaker
(6 to 9)% to Strength

, damage deals more ~50%, how to calculate this?
try with another source, the same result. Average 100% DTU/D deals ~ 125% damage more.

prolly best for devs to clarify it. I will mention DTU/DTD is the best scaling vector outside of sources magic damage to attacks