Can someone help me with the math?

Ok, so running full Lethality reds and getting the three main Lethality items, you get about 64.4 lethality

Precision gives 8.5 lethality (I could be wrong) so that's 72.9 lethality if you're running TLD. Going ferocity just gives 7% armor pen.

Let's assume our tank has 300 armor (lets have it be Amumu who at max has a base of 88.144 armor).

So with Tabi, sunfire, omen, and deadmans, we get 190 bonus armor. Add in armor scaling seals (not sure if that's bonus armor or not) and we get 215.92 bonus armor. Add Amumu's base and we get 304.064.

Now here's the math where I am more likely to fuck up.

If bonus armor applies first, then LW bypasses 123.3279 of the bonus armor, leaving 150.7341 armor for bonus. Totaling armor at 238.8781.

Now if BC does it's full shred, that total armor is 155.270765. After the mastery, it's 144.401816.

Now with the lethality runes and items, that brings Amumu down to about 80 total armor (if I did my math correctly). 80 armor (probably wrong) is 44% damage reduction.

So effectively a full pen build does well against tanks. Even when the reason why it was renamed lethality was because armor pen was a bad buy against tanks which made it falsely advertised.

I'm probably wrong here. Can I get someone with better math skills?