Air Question

[Chronus24]

It says all air magic has 25% armor penetration. While I loosly grasp this concent, I'm still not totally sure what it means. Could someone explain how this effects the damage. Thanks!

[Telastyn]

every 40 armor above the skill halves damage.

I'd imagine that 25% armor penetration would make that into only 3/8 damage reduction [or less if the penetration effects the total armor, not just the "advantage"] or... 25 damage on a 40 damage spell rather than 20 damage without the penetration.