So items like wits end and abyssal first reduce, than percentage reduction is applied, then magic percent penetration is applied so void staff and finally flat magic penetration like from runes or liandries
I'm asking because I've been trying a kayle build where I get both abyssal and wits end so wondering how the order works so far my calculations take 160 magic resist and turns it into 20
4th Chiming Lamb2 votesThe equation is RESISTANCE / 100 + RESISTANCE. At one point, I knew the entire formula down to % resistance deduction, but...I forgot it, and can't find it anywhere.
69982196_DEL2 votesWhat's the build?
Anyway, with only 
and
you will reduce/shred 45 MR from the target (whith full stacks). Kayle's passive is caculated each time you add a new stack of Wit's End. If you also buy 
it's passive will be applied before and Holy Fervor will stack multiplicatively with Void's %MR pen. After that all flat MR pen will be applied.
IDK if I answered your question, I didn't got it clear.
Edit: I was mistaken here. AS and WE reduce MR for 50, Holy fervor isn't applied multiplicatively with VS, HF is applied and then VS 's passive is fully aplied.
KoKoboto1 votesYeah you pretty much said the build lol, the only unique thing about it is that I get Abyssal and Wits end while usually people just get one of them for Magic Resist. So full build is like: 
I decided to use 160MR because on average a tank has 50~53 MR from natural scaling, 27MR from runes/18, than an additional 55MR and 25MR from 
&
.
I know percentages are applied first so first I reduce the MR but 15% from Kayle passive which turns 160MR -> 136MR next I reduce by Void Staff which is 136 -> ~84MR, and finally I take away 50MR with Abyssal and Wits End getting 84MR -> 34MR
69982196_DEL1 votesMan, you are doing it wrong. Holy Fervor applies a % MR REDUCTION, Void Staff grants a % MR PENETRATION.
KoKoboto1 votesMan, you are doing it wrong. Holy Fervor applies a % MR REDUCTION, Void Staff grants a % MR PENETRATION.
I know lol I'm doing the calculations based on just my champion so others will see the target with higher MR but to me I will see the target with 30
69982196_DEL1 votesI'm goibg to do the math for 160 MR and champion level 18, then you can find your mistakes:
160 - 25 = 135
135 - 25 = 110 (fully stacked)
Holy Fervor: 110 X 0.85 = 93.5 (fully stacked)
93.5 X 0.65 = 60.775 / 53.33 (if using rank 5 Piercing Thoughts)
If you have flat pen from runes, 
or 
you can rest their value from that total.
Now, Void Staff is barely gold efficient in that build. You may want to drop one of the flat reducction items (I think Abyssal is superior to Wit's End for Kayle) and replace it wih Liandry's. Also being a heavy AP focused build you should really try Death Cap instead of Rylai's. Although, to be honest, a Hybrid build is vastly superior for Kayle atm.
KoKoboto1 votesThanks for answering my main question which was the order of which magic resist reduction/penetration, flat/percentages are applied in. I originally thought that Abyssal and Wits end would be applied after.