In a somewhat recent post someone said the formula to figure out damage reduction was:

Let's take a target with 75 armor, a typical level 18 with no armor items.

75 / (75 + 100) = 43% reduction

Let's chop his armor off by 17 (just red runes):
58 / (58 + 100) = 37% reduction

Let's chop his armor off by 29 (red and quints):
46 / (46 + 100) = 32% reduction

Let's factor in Brutalizer for each value because you fail if you don't have brutalizer on most AD heroes. Going to skip writing the formula every time.

Nothing: 38% reduction
Reds: 30% reduction
Both: 24% reduction

Repeat the same thing with last whisper because a number of carry-type AD heroes will have last whisper. This is brutalizer and last whisper combined.

Nothing: 26% reduction
Reds: 21% reduction
Both: 16% reduction

The curve is not difficult to understand with different armor values. The less armor you have, the more effective armor penetration is. The more armor you have, the less effective it is. This is stuff that's well known already.

I think we can safely say that critical chance runes completely beat armor penetration if the above is all true because just reds and quints alone with crit will give you 14%.

Btw I think you can use 45 or so armor for level 10ish heroes in general (aka. no one who is stacking armor).

Quick version with brutalizer:

Nothing: 23% reduction
Reds: 12% reduction
Both: Basically no reduction because they have pretty much 0 armor.

So with red runes you gain 11% damage, and 23% damage with both runes. This looks good for armor penetrate but it's kind of trap. On the heroes you will buy last whisper on you should have both items at level 10.

45-15 = 30

Oops...your armor penetrate runes took their armor to 0, that means you gain absolutely nothing from the 40% armor penetrate on last whisper.

No math on the damage increase from crit? Kind of pointless in a comparison then

How is it pointless? You realize your crit percent is a direct increase in damage by that percent yeah? So in this case 14%.

Example:
You hit for 100, 100, 100, 100, 200 damage. Your damage is 600 with a 20% chance to crit.
You hit for 120, 120, 120, 120, 120 damage. Your damage is 600 with a 20% flat damage increase.

The difference is sustained damage vs spike damage but in the end if the RNG doesn't fail you will do the same damage in the long run.

I disagree with the statement that crit% beats out armor pen because 99% of the time the early game laning and ganking is what decides the game. That being said I find armor pen WAY more useful than crit% and I also prefer attack speed over crit% too...

The thing is it's mostly about early game and making the best out of the little amount of gear you have during the early phases and that is where I feel rune/mastery choices really shine, but if we're playing for end game max build then crit% will probably trump all because you'll have everything you need and just be that much closer to 100% crit chance with a green elixor pop...

Just wanted to point out that your assumption that every AD champ will build LW is bad. LW is only a good item if they are stacking some armor, otherwise there are better sources of AS out there (I generally seem to just swing into Starks). One of many things I find lulzy is when I see Ashe on the other team build a LW and no one on our team has stacked any significant armor.

Nidalee doesn't buy LW. Ever. Because of this full ArP Reds and Quints is better than Crit for her. Just popping in to defend my main "stacking" flat ArP.

Interesting. What happens if you go half and half?

When exactly did I assume every AD hero will buy LW? Right, never. I said on the few heroes who would buy it.

Nothing wrong with LW on Ashe. I buy it almost every game. Her AS is garbage and usually most teams have at least 1 person stacking armor.

Not tooting my own horn or anything but here's my results buying LW in 95% of the games I play on my first trip back.

http://i48.tinypic.com/35lalp2.jpg

You compare them with crit runes repeatedly in your example while including LW. If you are trying to point out that flat crit and LW are not as synergetic as people may think due to the order they are calculated in, I agree with you, but that has little to do with crit vs AP runes UNLESS YOU ARE GETTING LW REGULARLY. So either you implied something to which i replied to, or you are obfuscating the issue (wittingly or not) by including irrelevant or secondary information. Either way, I stand by my original rebuttal even if you did not explicitly state that all AD champs get LW.

You provide a lot of numbers, but no real comparison. But you got me curious, so I guess I'll do some number crunching over lunch.

you forget that armour pen works for abilities, whilst crit chance does not