How does Nunu's new W interact with deathcap?

Blood Boil (W) [New Effect] Now additionally increases AP by 40% for both targets

• Max: 40/60/80/100/120 AP

Does this bonus AP apply before or after Dcap is calculated? Does the 120 AP from W get multiplied by deathcap then that bonus double dips into his W?

Maybe the easiest way to explain this is with an example.

Say a Nunu with max W has 50 AP. He then buys a Dcap. This would bring his AP to 238 WITH the bonus from Dcap. Now his W will give him an extra 40% which is an additional 95. Does this additional 95 get multiplied by Dcap too? Because that would result in him gaining an extra 42 AP that would get factored back into the 40% from his W which would then get multiplied again by Dcap then recycled through his W over and over until he hits the cap on his W.

I understand this is a bit hard to follow but the easiest way of putting it is the multipliers double dipping off each other until the W cap is met.

So can any Rioter explain how his W would function if he rushed a Dcap?