I need help with math (AP Evelynn and magic penetration)

You can view my runes and masteries and stuff here:

http://matchhistory.na.leagueoflegends.com/en/#match-details/NA1/1801603168/207930793?tab=builds

So here's the issue. Even AP Eve does hybrid damage. I'm not sure what the ratio of my burst is, but if I were to guess, I would say around 66% magic and 33% physical (that seems to be what the end-game graphs say, but I'm not sure if it changes over the course of the game significantly or not). I'm also not sure how much that ratio changes with and without my ult.

So the problem: I was looking at this picture:

http://img2.wikia.nocookie.net/__cb20140221113726/leagueoflegends/images/d/d2/35perc_Pen_Dmg_Scaling.png

Which is on this page:

http://leagueoflegends.wikia.com/wiki/Void_Staff

And every little bit of math I did seemed to imply that if this was my final build:

Void Staff would be a better pickup for my burst almost 100% of the time, even against squishy targets, even if it only applies to 66% of my damage.

So the question: if I don't need the movespeed from a 2nd Luden's, is Void Staff always going to be a better pickup, even against squishy targets?

Things to take into account:

1. that graph in that picture doesn't include the mastery penetration

2. my assumed physical/magical ratios might be wrong

3. How many Qs am I gonna get off in a fight? How many spellblades? How many nashor proc's?

4. Don't forget about Deathcap when doing any math

Please help. If AP glass Eve is gonna be my new main, I don't wanna be doing stuff like this if it's wrong:

http://i.imgur.com/tdwQSjG.png