is there an armor breakpoint for the Last Whisper?

I'm wondering when it's better to buy another item instead of last whisper if your enemy has a lot of armor.
These aren't exact numbers but let's say malphite reaches 800 armor and has 85% damage reduction from physical damage. Getting a last whisper reduces his armor to 800 - 35% = 520 armor. That is still around 75+ % damage reduction.
Wouldn't it be more useful to get another item that increases your damage further? Since he has to stack huge amounts of armor, wouldn't a wits end be better?
Or the old madred's razor?
Best Answer
The League of Legends wiki page for Last Whisper shows this chart. It's still accurate.
It's hard to concisely describe the contents, but essentially, LW is more efficient for higher opponent armor values and lower total AD values.
This chart shows the increase in damage via the armor penetration on LW for values of armor.
Edit: The images are showing up small apparently, so here's the article they're from: http://leagueoflegends.wikia.com/wiki/Last_Whisper. The images are below the stat/availability block. Click them to see the full-size images.
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Can you stack last whispers?
Last Whisper editThis effect does not stack.What is last whisper good on?
Last Whisper is an item that increases attack speed (not damage), crit chance by 20 percent, and its critical strike reduces a target's armor by 75 percent for three seconds. LW is effective against comps that have an increase of armor, such as Vanguard.What does last whisper build into?
Patch History Making Last Whisper more effective as a component item while building into Lord Dominik's Regards or Mortal Reminder.Is Last Whisper unique?
Last Whisper now penetrates 33% of the defender's armor (Unique).Ghost Recon Breakpoint | How to EASILY deal with Behemoths
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