Vel'Koz 6th item: Void Staff better than stacked Mejai's on very low MR targets!?!?!

So I was bored at work and made some calculations for the optimal 6th item on Vel'Koz, and came to a startling conclusion. Despite having monstrous amounts of true damage, fully-stacked Mejais is only better than Void Staff if the target's MR is between 48 and 312.

The builds both used Morellonomicon, Sorc Boots, Ludens, Zhonyas, Rabadons, and Elixir of Sorcery with blue buff. I assumed a standard AP rune page and 12-18-0 masteries, taking Double-Edged Sword over Feast, Natural Talent over Vampirism, and Assassin over Culinary Mastery. Obviously, % increases from masteries and the magic penetration only affected the magic damage I dealt, not the true damage.

The combo I calculated damage for was Q -> W initial -> E -> Passive proc -> W explosion -> True damage ult -> Passive proc. I assumed one Thunderlord's proc, one apllicaiton of 25 true damage from Elixir of Sorcery, and one Luden's Proc (perhaps I should've assumed two since each tick of the ult would charge it?)

Basically what it came down to was this: Damage = Multiplier*Total Magic Damage + True Damage

Where the Multiplier was equal to the percentage by which the target's MR reduced my damage, times all of the percentage damage increases from masteries. As one would expect, against super-high MR targets, Void Staff > Stacked Mejai's since it increases how much magic damage the combo does by a massive amount. What was surprising was that if the MR is 48 or less, the Multiplier actually becomes greater than 1, meaning that EACH POINT OF MAGIC DAMAGE REDUCES THE TARGETS HEALTH MORE THAN EACH POINT OF TRUE DAMAGE.

TL;DR If you are rocking an endgame amount of AP on Vel'Koz (>800) Void Staff is better at bursting 0 MR item ranged champions than a stacked Mejais. Insane, right?