Percentage damage increase from magic pen! (Warning: Maths inside)

So if you run the math (Damage after/Damage before), the percentage increase in magic damage that penetration gives you is based on target resist r and your flat magic pen, p.

100p/(100+r-p)

So if you have 30 MR and LB hits you for 200 damage with her Q, after she buys sorc shoes she'll deal 232 damage with that same ability, the percentage increase is pretty much 16%.

When consider that later on you have 80 MR and LB gets Void Staff

(40r+100p)/(100+0.6r-p)

With the 40% pen from Void Staff and the 18 flat pen, her magic damage is increased to you by ~38.5%.

I recommend that you put both functions into Desmos as 4 "blocks":

y=100x/(100+r-x)

r=0 , click on the 10 or -10 and change bounds to 0