URL of calendar running on mac server (to subscribe in google calendar)
Subscribing to a Calendar and accessing a Calendar account are two very different things. As you mentioned above, you can access this calendar from a Mac by visiting "Mail, Contacts &amp; Calendars". However, to subscribe to a calendar there is a slightly more involved process.

First, go to your primary Mac that you use this account from. Open the Calendar or iCal app. Locate the calendar you'd like to access from Google from the Calendars sidebar. Right-click this calendar and select Publish... You can now enter a website U RL, user ID and password, choose your settings and click "Publish" to publish your calendar.

Take the URL that it published to and you can now subscribe to this published calendar in Google Calendar (via add via URL).

Keep in mind... when you publish a calendar, it essentially sits on a Web Server. That means you will need to have a web hosting provider to publish to. If you do not, you can always start the Web Service on your OS X Server and publish to that.