His W at max level gives him 40% Magic penetration for 8 seconds and void staff does the same with 35%. While I was playing him I saw on the character list he had 40% Magic penetration with masteries and void staff but when I hit W and checked it again it only had 64%. My math isn't great but 35+40 is 75 plus the other. Is there a cap on Magic penetration %?

There may be a diminishing return on the stat. accumulation, as I am aware that there is for most stats other than ad and ap. You have to take into account the diminishing return on armor and mr. If pen didn't also scale in such a way, you'd have a broken system.

But either way, void staff on Xerath is his bread and butter, most players do not realize. Void staff lets Xerath hit everyone hard regardless of how much mr they have; it secures his dps, while letting him pick up c/d reduction items earlier than most ap carries, which he also benefits from greater than most ap carries.

no diminishing returns as I understand but its not additive, its multiplicative.

The math, for your edification:

Xerath's W penetrates 40% of your opponent's MR, so you can say that they have 60% of their MR left

Enemy's MR: 60%

Now add on top of that the multiplicative stacking of Void Staff. Simply put, it reduces their MR by another 35% of 60%, so they lose another 21% of their total MR

Enemy's MR: 60-21 = 39% of original MR

subtract that from 100% and you get

100-39 = 61% MPen, or 61% of your opponent's MR that is not being used for damage calculation.

Not exactly 64%, but much closer to that than 75%... You might have also been running the %Mpen mastery, giving you another 8% MPen, which would calculate out to

8% x 61% = 4.88% of total MR penetrated. So it would be closer to 65.88% MPen.

I hope this was helpful, even if I still don't know how you got exactly 64% MPen.