Help me out! magic pen question

I have to post on this account cuz my main is 7 day banned for some comment i made about a vagina... anyway... im spiritoffizz, you can see my last game I built against a team with 2 huge tanks with zac's bonus magic resist 125 so probably like 200 total? and nautilus bonus magic reisst like 200 + whatever his base +runes and mastery so maybe 275.

Of course my strategy was to flank and wipe their carries as much as possible, but i also ran into fighting zac 1v1 a couple times(no wonder i had 58k damage...) and I wonder how effective my magic pen items +precision mastery was in breaking through his resist. Caitlyn had 0 mr and ori built a banshees veil late game... so I could blow them up super fast with all the magic pen making them pretty much take true damage from me. I wonder though if I would have been better off building a death cap and a gunblade instead of the pen items though?

Really my big question is though, how much damage was zac and nautilus mitigating though magic resist in this game? Syndra did like half my damage and she just had sorc boots for magic pen.