Seriously guys, if your gonna keep screwing with Armor Penetration and making it lower, make all of it reduction... this next patch has removed any desire for me to use Brutaliser or Youmuu's Ghostblade AND if your gonna drop them so low stop making them unique, no one can afford to stack that many anyways.
IE. 6x Blackcleaver would be 25% at max stacks and 60 pen, ops with 100 armor (pretty much base at 18) would be at (100*0.75)-60 = 15 armor left and that's still pen and not reduction on the chance to do -AR bonus damage. A Garen with 250 armor (250*0.75)-60 =127 armor left, leaves you the option to build on hit damage and hope he didn't buy MR.