Hi Wayne,
The short answer is - "the higher the better".
Longer answer: The farther away you can get the beam from any other interfering obstacle, be it trees, buildings, other antennas etc. etc., the more likely you are to have a pattern that is close to design. This is the case for ANY multi-element beam and
in general is NOT frequency dependent.
I seem to recall some discussions on minimum "useful" heights which suggested 2-3 wavelengths above ground.
So to return to the short answer - push it higher!
73 Andy