Lets do some yasuo math
Just something funny I wanted to look at. I am sharing it because I can
yasuo builds 2 crit items and goes 100% crit
reduces crit damage by 10%... this isn't a unique stat
damage reduction stacks multiplicatively though and yasuo has a -10% modifier on all his crits
so that's -10% + a possible 6 * -10%. Ninja tabi does 12% now that I look at it
so .9 * .9 * .9 * .9 *.9 *.9 * .88
actually if you build phantom dancer you can do 12% as well. but lets look at these other items first
.9 * .9 * .9 * .9 *.9 *.9 * .88 = .46
in theory, yasuo should be doing 46% of his total critical damage to you if you build

but since that's a bit.. strange lets convert this to a more cheese'd up build

.9 * .9 *.9 *.9 * .12 = .57
yasuo will deal 57% damage but
reflects premitigated damage.
310 armor plus the standard.. lets say 80 armor. That's 390 armor
Thornmail deals 15+ of his basic attack damage and deals another 25% of your armor as magic damage. so about 97.5, lets just call it 97
assuming yasuo hits for 1000 damage per hit,
1000 * 100/(100 + 390)
lets just assume he's penetrating something like 150 armor for some reason
so 1000 * 100/(100 + 390-150) = 294.117647059 damage per hit
but then there's the mitigation so
1000 * 100/(100 + 390-150) *.46 = 135.294117647
and then the thornmail will deal 97 + 1000 * .15 = 247 raw damage
but yasuo has 30 magic resist so 247 * 100/(100+30 + 40 from
and another 20 because crazy runes and masteries ) = 130 damage
He will do 135 damage to you
You will do 130 damage to him everytime he hits you
meanwhile your own autoattacks will be smacking him with 
its 1% of your maximum hp... you'll probably have at least 1000 but from this build alone you should have
= 450 hp
3x
= 1500
1950 * .01 is 19.5 then you add the 10 from base hp so 29.5 and then your actual aa damage, lets say 80 so 120 approximately
needless to say yasuo will be taking a pretty good beating from this if he goes crit lol
Crit damage is amplified auto damage. So even if someone takes 70% less crit damage, you're still taking that full auto damage + 30% crit damage
if you see what im saying. So the math for ninja tabi would be 12% reduce from an auto + crit, not 12% off a crit.
he gets half of half back... so just add 25% to all his damage after calculations for a rough estimate then... sounds good to me lol.
math, see if you can follow. League of Legends -