League-of-legends – When to max Essence Flux(W) over Mystic Shot(Q) on Ezreal

You counterjungle in several cases:

1) You know you can take their jungle without running into anyone. This happens most frequently when enemy jungler has just died or pops up on the map in another lane, giving you a window of opportunity to manipulate their jungle.

2) Your jungler/team is stronger, giving you the confidence to win fights even if you do run into someone. Examples of strong junglers are shyvana, udyr, and skarner.

3) If your opponent is absolutely dependant on certain buffs, like amumu on blue. You also contest their blue buff if their mid (like anivia or ryze) really needs it as well.

Magic resistance stacks additively, and is calculated via this formula (assuming positive MR):

Damage taken = Raw damage * (100 / 100 + MR)

25 magic resistance → ×0.8 incoming magic damage (20% reduction).
100 magic resistance → ×0.5 incoming magic damage (50% reduction).
source

Magic penetration is applied in this order:

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

Magic penetration stacks multiplicatively. Void staff (40% magic pen) and the Arcane Knowledge mastery (10% magic pen) together equal 46% magic pen. source

We can see that the types of magic pen are very important, because buying flat magic pen (sorcerer's shoes) makes % magic pen (void staff) less effective. Unless the enemy has a lot of MR (ex: Galio), it's not worth it to buy multiple types of magic pen.

Compare some examples with a base MR of 30. With 0 magic pen, 100 magic damage does ~77 damage. With Arcane Knowledge, we ignore 10% of 30 MR (27 MR total) and deal ~79 damage. With sorcerer's shoes, we ignore 20 MR (10 MR total) for ~91 damage. With shoes and arcane knowledge, we ignore 20 MR + 10% of 10 MR (9 MR total), or ~92 damage.

We can see that for low values of MR, increasing Magic Pen provides very little gain. It also shows how effective flat MR is vs low amounts of MR, whereas percent reduction is not.

Now let's consider a large MR, say 200 and 500 magic damage. With 0 magic pen, that's ~167 magic damage. Arcane Knowledge reduces the MR to 180, or ~179 damage. Sorcerer shoe's are the exact same here. With shoes and arcane knowledge, we ignore 20 MR + 10% of 180 MR (162 MR total), or ~190 damage. If we add a void staff to these 2 (discounting it's bonus AP for simplicity), we get 200-20-72-11 (97 MR total), or ~254 damage.

It's safe to say at this point that unless the champion is stacking MR, you'll do more damage buying AP instead of Magic pen. A notable exception to this rule is Soraka, because her passive provides a substantial MR aura.

tl;dr: Runes, masteries, and sorcerer's shoes provide enough magic pen vs most champions. Only buy additional magic pen if the enemy champion has over 100 MR.