incedibly inefficient.
% armor reduction comes before % armor penetration. the % armor reduction also scales MULTIPLICATIVE and not additive. which is why a lot of people in this thread get the wrong numbers.
cleaver on its own is often not the wisest choice now compared to lethality. the stat is going to get buffed next patch aswell.
- the spin shred + black cleaver add up to around ~47% armor shred on base armor and bonus armor making it a total of 53 + 53 in your case
- last whisper then penetrates 45% armor off the 53 bonus armor left, making it 53 + ~29
last whisper + black cleaver + his spin shred in your scenario would reduce the enemy armor down to about 82.
against really tanky teams, who stack massive amounts of armor this is something you can do, if you want. though against squishier targets, last whisper will add almost nothing to your build.