this post is so funny...

anyways

1 armor / mr increases your effective health by 1%

100 armor increases your effective health by 100% meaning its at 50% damage reduction

+1 boots are 50, +2 boots are 70 + 3 boots are 90 and the +6 from mobillty, if following the + 20 thing, should be atttttt +130

for armor / magic reductions it goes

debuffs (black cleaver, fiddles passive, etc) - CAN LOWER BELOW 0
% reduction (void staff, last whisper) - cant go below 0
armor / magic penetration (runes, abyssal, brutalizer etc) - cant go below 0

% reduction works on the value they have, not %

if they have 100 armor (50% reduction) a lw wont make it 10%

it will make it 60 armor which would be about... lets say 35% (dont remember the calculation)

For Armor and Magic Resist:
The reduction formula is Armor/(100+Armor) = % Reduction. So 100 Armor is 100/200 = 50% reduction. Armor protects against physical attacks, MR protects against abilities and effects that do magic damage. There is no overlap (so having 100 armor and no MR does nothing against spells)

For Run Speed:
Enhanced Movement 1 is +50 to your movespeed.
Each point of Enhanced movement beyond that gives +20 to your movespeed.
% Movespeeds are then applied to the result of your base move speed plus your Enhanced Movement Bonus. However, there are Soft caps at 420 and 490. Any movement enhancements over 420 are reduced by 20%, and any movement enhancements over 490 are reduced by 50%.
Example: You have Jax (330 base move speed) with Boots of Swiftness (Enhanced Movement 3, which translates to +90 move speed). This gives you a total speed of 420. You then have 3% movespeed from your masteries and 8% from a Zeal. So you add 11% move speed to 420, which would give you a bonus of 46.2 move speed. However, since this exceeds 420, it is reduced by 20% to 36.96. So your end Move Speed is 457.


Some other common questions:

Dodge is Multiplicative. This means each chance to dodge is only applied to their remaining chance to hit after your other dodge effects. So if you're Jax with his top-level dodge ability (18%), 2% from masteries, 8% from Runes, 20% from a Phantom Dancer, and 11% from a Ninja Tabi, this doesn't give you a total of 59% Dodge. Instead, it gives you .82*.98*.92*.80*.89 = 52.6% chance of being hit, or 47% Dodge.

For Armor/MR Penetration and Reduction: Reduction (effect like Black Cleaver, Stark's Fervor, Abyssal Scepter, and various abilities) applies first, and can lower into the negative. Negative values of Armor/MR increase the damage by the same % that a positive MR/armor would reduce it by. After Reduction, if Armor/MR is positive, % Penetration (Void Staff, Last Whisper, Archaic Knowledge Mastery) applies. After that, Flat Penetration (Sorcerer's Shoes, Runes, Haunting Guise, Brutalizer, Sunder Mastery) applies, which cannot go negative.