That means that on a pre-elixir armor reduction value of 50% (not that hard to get) is same as 1.4% overal reduction.
At 55% pre-elixir it's ~1.5555%
At 60% that would be 1.75%, although with DR it'll be closer to 1.7%
So let's just say it's an average of 1.5% physical damage reduction.
90 resistance on top of the 195 you should have is an average of:
1 - [(1 - R(285)) / (1 - R(195))] = 1 - 71.75 / 78.78 = 8.92% magical reduction
Portion of damage intake to have them be equal:
1.5% * (1 - MP) = 8.92% * MP
1.5% = 7.42% * MP
MP = 16.82%
Or: the magic damage you take needs to be ~1/6th of the total damage.
Assuming ~20k melee DPS on you, that's 4k magical DPS or a 80k magical hit pre-resistance every 20s.