Talk:Magic penetration/@comment-85.210.28.75-20120127200244/@comment-4087140-20120128000458

Flat penetration is calculated before percent penetration, and percent penetration cannot reduce a target to 0 (because it is impossible to get 100% magic penetration).

Without any runes, a 30-MR target would be reduced to 10 from, and then reduced by 10% from down to 9. However, to actually reach 0, you would need a total of 10 magic penetration from runes.

tl;dr 10.