Nest Thermostat contol of Sentry Boiler

I have a Sentry Gas Fired Boiler (Model S-90 EDP) with an old battery powered Thermostat currently hooked up via a R and W wire directly to the T & Tv wires as showing on Page 7 of this installation manual:

I am wanting to install a Nest Thermostat and have an unused 3rd wire running between my Boiler and my Thermostat. Where would I hook up the spare wire to Boiler in order to use it as a Common wire (C connector) on my Nest?

Thank you in advance for your assistance. [IMG]