I normally look at the season and last 10 games. The reason I look at the last 10 or so games is to look for any type of trend. Personally, I place more weight on the more recent games as I believe it better reflects the way the team is currently playing.
As far as the way the averages are calculated that is a trade secret. Just kidding, I usually don't have the time to put these averages together myself so I just pull them off of various sites such as VegasInsider and phoenixsports (props to Monkey for this site).
Hope this helps.
If you have any additional input let me know.
Good luck