The Auto settings in Encore often leave empty space on the disc that could be used for a higher bitrate/quality.
A couple of guidelines I use;
1.Dont exceed 8 mbps video bitrate.
2. Fill up the disc with the highest video bitrate without voiding #1.
3. 2 Pass VBR for movies over 75 minutes.
Calculating bitrate involves a bit of math. The number of seconds in your movie times the the number of video bits per second and then divided by 8 to convert to Bytes should be close to 4 GB.