User blog comment:Rapacious/Rapacious's sexy guide to understanding ranged AD builds/@comment-86.41.204.25-20120201095349/@comment-3238314-20120201103114

I mentioned having full armor penetration runes for that, which is 31. 31 + 45 is 76. if 76 is 40% or less of the armor penetrated, that means that the armor has to be above 190 armor for it to be better than last whisper with no armor penetration runes. However, if you have 31 + 40%, then the value drops to about 160 armor, which is the rough estimate i gave.