Handbrake's help files say that SD sources should have the Quality slider set between 19 and 21, while HD sources should be set between 21 and 23 (lower is better picture; higher is smaller file size).
They say that at some point you reach diminishing returns with the quality setting, so setting HD sources to 19 may not be the most efficient setting.
http://trac.handbrake.fr/wiki/ConstantQuality