What terminal to use on AC for C wire / nest dual transformer install
This is your common 
You connect here and bring it up to the stat with the spare wire from the boiler. 

Nest is also capable of power stealing in many cases. Here's a link to check before running a new wire. 

https://nest.com/ca/support/article/When-Nest-needs-a-common-C-wire#wire-you-checker