Board Thread:League of Legends discussion/@comment-24603390-20140317093853/@comment-24603390-20140404222229

These calculations show when it is efficent to have Last Whisper. It allows you to determine when you should purchase Last Whisper in a build order. If you are facing opponents with low armor, it is more efficient to purchase more AD items instead. This explanation was originally on the Last Whisper page, but was moved here, and replaced with a simplified version on the Last Whisper page.