The only sure fire way of measuring is to time from start to finish a normal day's usage. From firing up a youtube HD video or watching a movie, doing some background downloading, torrents, etc, etc.
When watching hulu 1 hr episode goes down to shy of 4 hrs 3/4 brightness to appreciate it. But yes, you are right, I dont have time to cover all variables I'll leave that to the younger crowds. For my purposes, writing/browsing/email is 90% of what I need while on battery charge. Watching TV or playing games I just do with power cable