leveling a mage through pvp, spell pen question

[tyrant21]

So I just recently reacquired my original toon, a 60 mage who i have since leveled up to 73, in any case, I am mostly leveling him through bg's because I am sick of questing and really enjoy bg's/
In any case, I have noticed a fair amount of my spells are being resisted at this level, what kind of spell pen would i need at th73 and as i level up through the 80's?

[Goliethlol]

You should be aiming to have as much spell penetration as is available from a paladin's resistance aura or shamans resistance totems at your given level.