I think this is is how to Equality set works: if you manage to get 1000 HP and 1000 MP, that's 100% equality. Since it gives 37.5%+ extra dmg , this means at 100% equality, it's guaranteed 37.5% extra dmg. Of course to get 100% Equality easily, the Equivalence set is for that and I think that was the intentions for that mythic in its inception. Also at 100% equality means guaranteed 37.5%+ regen. That means 1000 MP regen get multiplied by 1.375.
Even if you're 90% equality, it's good. If you have 26k hp but 52k MP, that's 50% equality meaning the 37.5% dmg boost turns into 18.75% dmg and regen buff.
Equivalence reduces dmg by 60% but only when you're at full HP and full MP and not taken any dmg as of yet. If you have 50% hp taken off you , then the Equivalence set reduces dmg by 30% or 45% instead of 60%. Also the mythic works so if you have 100k HP and 10k MP, it equalises so you have 100k HP and MP , give or take.
Equality is quite a good set for sure , especially in a build that uses Equivalence. Can be good in PvP.