Aside from the part where the cap is /3 which is ~30%, as is what the tooltip states your max to be... yes you have it correct. It's divided by 3 (likely 3.33 really) in order to max out your penetration at %30 of a mob's armor.

So roughly 400ish armor penetration will cap your ability to penetrate a L83 mob.

This is COMBINED with Sunder and other armor reducing things, so you're going to get more from less if someone else is debuffing the armor of your target as well as your gear doesn't appear to be able to hit the full %30.

Whether this is as useful as adding to haste or other stats... that's a different discussion. I think this does in fact prove that armor pen isn't useless on your gear, but you still aren't going to want to gem for it.

You can never ignore all of a target's armor. That's the reason why the calculation is done the way it is. All LK bosses have 13083 armor and the max or cap that can be ignored works out to 9906. Say you have 30% ArP rating. That would make you ignore 30% of the 9906 or 2972. Take that number from 13083 and you have 10111 for the boss's armor. A reduction of 22.7%, not the 30% given in the tooltip. 13083 armor provides 44% damage mitigation. The reduced number of 10111 works out to 37.8% mitigation. Therefore you will do 6.2% more damage with your physical attacks which is not bad.

In my case, when my Grim Toll trinket procs I have 728 ArP or 59.2%. Working out all the formulas gives me 13.8% more physical damage for the duration of the proc. That's pretty substantial.

ArP is definitely not the useless stat some people think it is.

Unless you do some damage that isn't physical, like ret pallies and DK's. For us it still completely sucks.