What kind of beer are you dispensing?
In general I go by this:
British Ale: 1.8 to 2.2 volumes
German Lager: 2.5 volumes
American Lagers and Ale: 2.6 to 2.8 volumes
Wheat Beers: 3.0 volumes
Then you can use that calculator that is posted all over here
Hope I wasn't useless
-Seth