Before buying a Void Staff ...

Be sure the enemy team actually have at least one MR item each (or at least 3+ do), or it just won't be worth it. I see so many people buying a Void Staff when no one on the enemy team (besides maybe the tank) has a single MR item. That's quite a bit of wasted gold.

The percentage magic penetration stat must have at least a value of 772g for Void Staff to be gold efficient, since the item costs 2295g and the 70 AP it gives has a value of only 1523g. The 2295g value occurs after 64 magic resist, (a 22.4 flat penetration) which is worth 769g, is taken into account. This means the enemy would have to have a minimum of 65 MR before the gold value in Void Staff begins to give you a profit.

In laymen's terms, you're basically paying 2295g for an item that is practically weaker than Needlessly Large Rod (which is 1600g) if the enemy doesn't have the MR for the passive to properly effect.

Something to note, if you don't know yet, is that all ranged champions have an MR of 30 that does not improve with levels. Everyone else (melee champions) who do get MR with level growth (which is 1.25 per level) will be sitting at 53.4 base MR by level 18. A list of everyone's MR can be found here: http://leagueoflegends.wikia.com/wiki/List_of_champions%27_magic_resistance

Some math:

Sorcery Boots: 15 M.Pen M.Pen Reds: 7.8 M.Pen (5.5 with Hybrid Reds) R3 Devestating Strikes: 6% M.Pen (and A.Pen)

Most AP Carries should have these 3, at the minimum. By default, you'll already have 24 M.Pen with this simple set-up. (20.7 with Hybrid Marks.)

Liandry's Torment: 15 M.Pen Abyssal Scepter: 20 Magic Resist Reduction Wit's End: 0-25 Magic Resist Reduction (depending on stack count)

It's important to note that Abyssal Scepter and Wit's End are not M.Pen. They are Magic Resist Reduction effects. It's important, because M.Pen can not reduce MR below 0, but Magic Resist Reduction CAN reduce below 0, which gives bonus damage. However, I was shown that the order of calculations makes it so Magic Reduction goes before Magic Pen, so this would be difficult unless a team comp specifically tried to make this happen.

Now, let's look at MR the enemy can have outside of items.

Flat MR Blues: 12 MR Scaling MR Blues: 27 MR (at level 18, mostly seen on Junglers and some top laners) Ranged Champions: 30 base MR Melee Champions: 32.1 - 53.4 base MR

Unless the person is a tank and puts 21 points into the Defensive Mastery tree, this is the most people will have for MR when it comes to anything outside of itemization. Realistically, most of the players will have 42 MR total.

If they don't have any notable MR items, you'd be better off with a Liandry's Torment or Abyssal Scepter to combat the rest of the MR instead of a Void Staff. In this Cinderhulk tank meta, Liandry's Torment is very powerful with the passive it has. Liandry's Torment is practically Cinderhulk's counter for AP Carries.

Edit: Had wrong data. Fixed it. (Same thread as the one in GD. Sharing it so it sees a broader audience.)