Skinrai/Vashi Damage Debuff Calculation

elium85

Xbox player here so a little in the dark about an effective way to calculate this but here goes. Does anyone know how the damage debuff from the shadow/light mark during the twins fight is calculated. If I'm marked Light and and attack Skinrai, how is the amount of damage I am doing actually calculated. It seems to differ between instant abilities, weapon attacks, and channeled abilities.

Thanks!