Talk:Magic penetration/@comment-77.249.233.124-20120330125906/@comment-4087140-20120330150913

Flat penetration is most damage-effective against targets with low resistances. Consider having 20 magic penetration versus two targets, one with 30 MR and one with 100 MR. As you can see, flat penetration actually has a smaller overall effect on your damage output as the target's resistances increase. I hope this helps you in choosing your runes.
 * 3020=10 effective MR, so the target has 110% effective health instead of 130% effective health, this means you deal 130110118.2% damage against the target.
 * 10020=80 effective MR, so the target has 180% effective health instead of 200% effective health, this means you deal 200180111.1% damage against the target.