Runes, as well as items like Sorcerer's Shoes and Haunting Guise offer FLAT penetration (i.e., you ignore up to X) of your opponent's MR.
The "average" champion has a base MR of 30, that doesn't increase with levels. With ~9 MR penetration from runes, sorcerer's shoes are enough to bring that down to basically zero.
The other options are reduction (triggered by champion abilities like Amumu, Ryze, or Fiddlesticks, or the Abyssal Scepter) which actually decrease your opponent's MR (and so stacks with penetration) and % based (the talent and the void staff). Of these, Reduction is the only way one can bring your target's MR below zero.
When a single character benefits from one or more of these, they are applied in this order:
- Flat Armor/MR Reduction
- % Based Armor/MR Reduction
- % Based Armor/MR Penetration
- Flat Armor/MR Penetration
Some of the above information can also be found in the game mechanics thread on the official LoL forums, here: http://www.leagueoflegends.com/board/showthread.php?t=3287
The answer to your question would then become:
54.36 MR * .85 = 46.206 MR - 8.55 = 37.656 effective MR. (I don't know the formula off hand to convert that to % damage reduced)
LoL has some inconsistent wordings, I'll agree.
As I mentioned in the comments, you're looking at the wrong aspect of armor. You shouldn't be looking at the percentage it reduces, but rather the amount of damage it actually prevents as a measure of how much longer that armor lets your survive.
Let's consider an example for a champion with 0 armor. Say he has 1000 health, just 'cause that's a nice round number. Now our champion gets caught in Garen's judgement, and starts taking 100 physical damage each second (at one hit per second). Clearly, our protagonist can take ten hits before dying.
But now, imagine our champion has the 50 armor you mention, meaning the incoming physical damage is reduced. Now (still with 1000 health) it takes 15 seconds for that bastard Garen to claim our champion's life. 50 armor extended our life by 5 seconds. (100 * (2/3) = 66.6 DPS; 1000 / 66.66... = 15 hits)
So our champion respawns, wises up, and buys 50 more armor. Now with 100 armor, we're sitting at a flat 50% damage reduction. Again we run into Garen, who's still dealing 100 base damage per second. It now takes him twice as long to kill us -- 20 seconds. Once again the armor has extended our life, again by 5 seconds. (100 * .5 = 50 DPS; 1000 / 50 = 20 hits)
Back in base, we buy another 50 armor. We're now at 150 armor, and 60% damage reduction. Enter Garen -- spinspinspinspinspinspin and it takes 25 seconds before our champion dies. Again, adding 50 armor extended our life by 5 seconds. (100 * .4 = 40 DPS; 1000 / 40 = 25 hits)
So even while the amount of damage reduced per point of armor diminishes (as you observed), the amount of time armor extends your life (or effective health) remains distinctly linear.
Best Answer
Every champion in League of Legends has armor and magic resistance stats. Even marksmen, who typically don't build any defensive items other than Guardian Angel or Mercurial Scimitar, have some base resistances that reduces damage.
When you buy defensive items, you are often getting additional armor or magic resistance stats. This additional value is called bonus resistances.
Last Whisper (and its subsequent items) penetrates only bonus armor. That means that base armor is ignored.
For example, using completely fictional numbers, let's say we have a champion that has 10 base armor that has purchased 90 armor, for a total armor stat of 100. Last Whisper will reduce his armor rating by 27 (30% of 90), leaving his total armor stat at 73. As another example, a champion with 90 base armor and 10 bonus armor will only have his armor rating reduced by 3 (30% of 10), leaving his total armor stat at 97.
Void Staff is a little different - it ignores 35% of the total magic resist. So using the two sample champions from before, no matter what their ratio of base vs. bonus magic resist is, they would both have their 100 total magic resistance changed to 65.