Mage damage shield question

[MikeXG]

So in theory, if a mob had 1000 HP - The DS somehow did 800 DMG you would only need to do 101 damage while you pet did the remaining 99 to get full XP? Even though in this scenario you didn't actually do half the mobs HP? Or Do you need to do 51% of the damage regardless of whatever makes up the remaining 49%?