User blog comment:Rapacious/Rapacious's sexy guide to understanding ranged AD builds/@comment-84.113.65.130-20120420124242/@comment-68.52.108.241-20120830162810

If they're stacking armor, you need the Last Whisper, because it lowers a percentage.

If they only have a bit but its enough to matter, you need Black Cleaver, because it only shaves off a flat amount.