User blog comment:Krufix/Armor/Magic pen is anti health stacking./@comment-1153246-20130725213506

Your math is wrong, unfortunately. Assuming a base armor value of 30, a Chain Vest brings you up to 70 armor, which, by your numbers, is 70% more effective health against physical damage, which brings you to 3400 health. Buying flat health items takes into account your 30 base armor, and as a result, buying even a single Giant's Belt brings you to 3k health base, and a 30% bonus on top of that brings you to 3900 health.

With that said, Qætz has the right of it, with his comment about proper effective health calculation. But yes, your basic gist is correct, you should definitely have some manner of pen against high-health targets, if for no other reason than the fact that almost no one stacks JUST health; they usually back it up with ~150/150 resists to make sure they don't get gibbed by things like that.

(The easy answer to health stacking is Vayne. XD)