To help clarify the question because the title doesn’t seem very clear, here is some context:
Reuniclus (Magic Guard) vs Durant (Hustle)
Reuniclus used Calm Mind! Durant used Toxic! (Reuniclus has the toxic effect but takes no damage)
Reuniclus used Calm Mind! Durant used Iron Head! (Does minimal damage)
Reuniclus used Calm Mind! Durant used Entrainment (moves hustle onto Reuniclus)
Reuniclus took toxic damage (?%)
Basically, if Magic Guard is taken away a couple turns after being badly poisoned, will the damage be high like 18% as if Reuniclus had been being damaged for that amount of turns, or would it start at 6% like normal toxic damage?