We get 30 Tenacity from Runes and 70 Tenacity from our items and boots.
ofc thats not gonna be 100% Tenacity in the end, since it would be broken bc u couldnt be cc`ed anymore.
Tenacity stacks by multiplication and not addication.
Here's the maths:
1*0,1 = 0,1
0,1 + 0,9*0,2 = 0,28
0,28 + 0,72*0.3 = 0,496
0,496 + 0,504*0,2 = 0,5968
0,5968 + 0,4032*0,2 = 0,67744
Final tenacity: 67,744 %
riot converts those stats into 70% tenacity (i tested it)
if somebody likes to know the formula or doesnt understand it , just write a comment and ill explain it.
but yeah, getting 100 Tenacity gives u an effective tenacity of 68
so that means cc is 68% less effective on u.
imagine being cc chained by morgana for 4 seconds
with ur 68 tenacity its gonna be only 1,28 seconds
sounds broken right?

im a psychopath , right? so i will play this build every game even if theyre full ad I WILL BUY MERCS. theyre broken in my eyes.
since u start with krakenslayer/rageblade first 2 items, u have a corebuild u can go every game. wits end and steraks provide many defensive stats (mr,hp,tenacity) and alos provide u with damage (bonus ad from steraks and onhit magic damage from wits end). u also gain tons of atk speed from wits end, which synergyses insanely well with rageblade and lethal tempo.
and my last item recommendation is to go for deaths dance or guardian angel. u wont get bursted anymore and u finally get a little bit of armor.
if u play vs full ad or u need armor really badly, u can go kraken -> rageblade -> deaths dance . i would STILL go mercurys, but its UR call. and after deaths dance, jsut go steraks into wits to get ur 100 Tenacity build finished :)