Your calcs are a bit off...
Resistance formula for converting armor/MR into %-reduction is 100/(100+Amor or MR).
For example:
If the enemy has 95MR and 1k HP, and you only get Sorc's (leaving the him with 80MR)
He would have a 100/100 + 80 = ~0.55 | 1 - 0.55 = 0.45
= He would have 45% dmg reduction, leaving him with 1000+450 effective HP.
Resistance formula for converting armor/MR into %-reduction is 100/(100+Amor or MR).
For example:
If the enemy has 95MR and 1k HP, and you only get Sorc's (leaving the him with 80MR)
He would have a 100/100 + 80 = ~0.55 | 1 - 0.55 = 0.45
= He would have 45% dmg reduction, leaving him with 1000+450 effective HP.
No that's not how it works lol.
To make you realize how silly it is:
Using your way to calculate ''Effective HP'':
1000 HP + 20 Armor =
100/120 = 0.8333 | 1 - 0.8333 = 0.16667
1000 HP + 16.67 % = 1166.7 EHP
1000 HP + 900 Armor =
100/1000 = 1/10 | 1 - 1/10 = 9/10
1000 + 90% = 1900 EHP
So you bought 880 Armor and got 733 extra Effective HP. Sounds tooooooooootally legit :P.
The actual real way to calculate is:
1000 + 20 Armor = 1200 EHP
1000 + 900 Armor = 10000 EHP
Resulting in, 880 extra armor = 8800 extra EHP.
http://leagueoflegends.wikia.com/wiki/Armor
To make you realize how silly it is:
Using your way to calculate ''Effective HP'':
1000 HP + 20 Armor =
100/120 = 0.8333 | 1 - 0.8333 = 0.16667
1000 HP + 16.67 % = 1166.7 EHP
1000 HP + 900 Armor =
100/1000 = 1/10 | 1 - 1/10 = 9/10
1000 + 90% = 1900 EHP
So you bought 880 Armor and got 733 extra Effective HP. Sounds tooooooooootally legit :P.
The actual real way to calculate is:
1000 + 20 Armor = 1200 EHP
1000 + 900 Armor = 10000 EHP
Resulting in, 880 extra armor = 8800 extra EHP.
http://leagueoflegends.wikia.com/wiki/Armor
I really like the item on Amumu. In combination with Rylai's Crystal Scepter, Sunfire Aegis and Abyssal Mask, it makes him the ultimate nightmare for the Warmog's Armor people.
Edit: Scratch that.
The formula I used earlier was indeed correct, but I did calculate wrong there...
If you have 45% dmg reduction you don't get 1000+450 effective HP, but nearly 1000 x 2.
If you got 90% it goes like x10.
My fault, you kinda confused the heck outta me with that wiki calculation style.
The formula I used earlier was indeed correct, but I did calculate wrong there...
If you have 45% dmg reduction you don't get 1000+450 effective HP, but nearly 1000 x 2.
If you got 90% it goes like x10.
My fault, you kinda confused the heck outta me with that wiki calculation style.
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But yeah I agree it's very underrated... I would definitely not get it on Akali because I think there are much better items to get, but on most other casters I agree it's usually strong.
However... I want to say something to you, EvilDive, and many other people out there:
Let's say an enemy has 40 MR and you have Sorcerer's Shoes. It's like the enemy actually has 25 MR. Let's say he has 1000 HP and you deal 350 damage per second (but after a few seconds you can't do anything for a good while). The enemy has 1000 + 25% = 1250 Effective HP. You will kill him in 1250/300 = ~3.57 seconds.
Let's say you have Haunting Guise this time (Ignore that fact that it gives extra AP)
5 MR = 1000 + 5% = 1050 Effective HP.
1050/350 = 3 seconds to kill him
3.57 - 3 = you killed the enemy 0.57 seconds faster.
Now let's say the enemy has 95 MR and 1k HP again.
With Sorcerer's Shoes but No Haunting Guise: 1000 + 80% = 1800 Effective HP
1800/350= ~5.14 seconds to kill the enemy
With Haunting Guise: 1000 + 60% = 1600 Effective HP
1600/350 = ~4.57 seconds to kill an enemy
5.14 - 4.57 = You killed the enemy 0.57 seconds faster.
There we go. The conclusion is that it doesn't matter how much MR your enemy has, 20 Magic Pen is 20 Magic Pen and it will always be just as strong. It doesn't depend on the enemy's MR, it depends on your champion's BASE MAGIC DAMAGE values on his/her kit.
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