I prefer my knees just slightly under the piano.  Nobody has mentioned
that to play properly, your elbows have to be the same height as your
hands, a little higher doesn't matter too much, but never have your
elbows lower.  We are not all made the same height and -- old piano,
new bench (none adjustable), old or new castors on piano -- there are
lots of variables, but if you stick to this basic code "hands and
elbows", it is a start to better things.

Steve Bentley
Vancouver BC