Did some Lethality math.
Had some fun with Lethality math. This is purely in the spirit of flat reduction, not %.
With a:

You can get 60 Flat Armour Penetration vs a lv 18 target.
With Marks & Quints of Lethality as well as the Precision Mastery you can push this all the way up to 92.5 Lethality.
Why stop there though? We have some champions we can throw into the mix.
's E can reduce armour by up to 40.
's E can do the same!
isn't one to be left out though and can reduce armour by a flat 30 with his taunt.
Y'know, just in case you wanted to draft a comp that can ignore 202.5 armour for a few seconds and (only) 92.5 the rest of the time.
Figured this post would amuse someone as much as it made me cry.
Edit: My bad guys, throw in
for an extra 10 armour pen. That totals us to 212.5 flat armour reduction. Remember though, for the 50% of tanks that actually buy more than that % reduction is available to purchase in 4 different forms and one of those (aka the component for 2 others) is getting buffed! Remember all that % applies before flat so with just a last whisper they will soon have to have over 308 armour for you to not reduce them to 0 with just that. Currently you actually have to upgrade the item though.
Also is it just me or is it sad there are more lethality items than there are champions with flat armour reduction in their kits?