Last whisper is supposed to be gold efficient vs tanks, correct? If you use it against, say, a target who has built no armor, it would be assumed to be a highly inefficient item.
lol wiki Values each point of AD at 36g and each point of armor at 20g. One can assume then that each point of armor you ignore will be worth 20g.
Via her base stats, Caitlyn has 85.8 armor. Hands up anyone who uses flat armor yellows on their ADC rune page? That puts Caitlyn at 94.8 armor.
Ignoring 35% of that gives you an effective 663.8 gold. Plus 40 AD, that's 91.4% gold efficiency against a target building ZERO armor items.
Why does our niche armor penetration item give us 91% efficiency against the squishiest, least armored class in the game, Riot?