Magic question

...yes, I'm looking at you, vicalis -- but if anyone else has the answer, feel free to sing out!

Can someone please point out the number in the Comprehensive Rules that shows how the converted casting cost of X is treated while a spell is on the stack? Specifically:

I understand that Chalice of the Void, while in the library, has a converted mana cost of 0.
I thought that Chalice of the Void, when cast with 2 mana and sitting on the stack, still had a converted mana cost of 0. I'm told it's 2, but that doesn't seem to be what the rules support, and the person I would normally ask isn't available.

Thanks.