Not Updated For Current Season
The masteries shown here are not yet updated for the current season, the guide author needs to set up the new masteries. As such, they will be different than the masteries you see in-game.
with this build you will end up with:
347 magic resist
and becouse your passive gives you 50%of your magic resist in AP,
in this case it will be: 173 extra AP.
but with these masteries your shield (W) gets 13,2% stronger, this means that
instead of 90 armor and magic resist you get 101.
with as result when you use your shield:
449 magic resist
and with your passive 224 extra AP....
and THAT is what i call a REAL AP TANK!