Please note that the following analyze is based on the actual Leblanc's Full Build, which includes the 9 runes Greater Mark of Magic Penetration (MPen: 7.8), and the Mastery Arcane Knowledge (MPen: 8%).

Before we begin, you have to know that magic penetration and magic reduction are considered on the target champion in the following order:

1. Percentage magic reduction
2. Flat magic reduction
3. Percentage magic penetration
4. Flat magic penetration

Furthermore, here's a little legend to understand the rest of the post:

DM: Damage Multiplier
MPen: Magic Penetration
MR: Magic Resist

Red: It's the closest amount of MR that will give you a 100% DM


(Think that you are watching your amount of MR, and...)
The first column (MR) is the amount of MR you have.
The second column (DM) is the Damage Multiplier from incoming damage if other champion has MPen: 0 | 0%
The third column (MR after Pen) is the virtual amount of MR you would have remaining from another champion's Magic Penetration hitting you.
The fourth column (DM + MPen) is the Damage Multiplier from incoming damage including the Magic Penetration of another champion hitting you according to the amount of MR you have.

(Now, we watch another champion's MR...)
Let’s say you begin with your Boots and 3 Health Potion, which means you currently have MPen: 7.8 | 8%


After recalling / dying a couple of times, you can buy your Sorcerer's Shoes. You now have MPen: 23.8 | 8%


The games goes on, you should now have something like a Rabadon's Deathcap or Rylai's Crystal Scepter, a pro like you knows that you need Magic Penetration, but why take Void Staff instead of Haunting Guise? Allow me to show you the difference by beginning with the Haunting Guise, which means you would have MPen: 37.8 | 8%


Now, if you take Void Staff, which means you would have MPen: 23.8 | 43%


Finally, you decide to buy one of those two previous items ( Void Staff or Haunting Guise) you didn't bought. Consequently, you achieve to have Sorcerer's Shoes + ( Haunting Guise OR Liandry's Torment) + Void Staff with MPen: 37.8 | 43%


In conclusion, if you are wondering which item you should buy between Haunting Guise or Void Staff, here's my advice: if you see that the enemy champion has low MR (like 20 for example), you can try with the Haunting Guise, it will be more effective, even if he attains level 18 (if it is a champion wih 30 MR level 18), until he buys some MR. But all other champions must have at least a little bit MR and it is enough to make you prefer Void Staff. So if the champion you plan on killing has few MR, go Haunting Guise; otherwise, Void Staff all the way. You can still, later on, buy Haunting Guise, it is now viable since the last update.

Warning: You can't go buying always a lot of AP and only AP, because when the other champions are level 18, they have at least 30 MR, and with aegis, 40 at minimum. Consequently, your total damage would always be reduced by at least 25%, or up to 40% if targetting a tank early game. Same thing if you go on with MPen and only MPen, it's funny you will be doing true damage, and even multiplied damage, but your lack of AP won't make you powerful enough to benefit the Penetration.


Algorithms:
If MR > 0 .. Damage Multiplier = 100 / (100 + MR)
If MR ≤ 0 .. Damage Multiplier = 2 - 100 / (100 - MR)