Hi John,

Sounds as if you've sorted out most of the Occult and Guide aspects
of the situation. If you're going to attempt to get really good
occultation predictions with Guide, you should take a look at

http://www.projectpluto.com/ast_occu.htm#do_it_yourself

(And some of the basic ideas involved may be of interest even if
you're using Occult instead.) As you'll see, it's not exactly a
straightforward process. You can get pretty good results by just
clicking on the asteroid in Guide, then on the target star, and
then on "Show Eclipse". But this will be an orbit based on generic,
medium-precision astrometry and for an epoch that can be up to 100
days out of date, ignoring the perturbations over that interval.
The way to get around both problems is described at the above link
(as are ways of making the charts look better and be more useful).

In this particular case, I suspect that 5 September is far enough
off that an update has not been produced yet; the IOTA prediction
is probably based on older observations, and will change a bit as
new astrometry is used. (As would Guide's, were the same orbit
fed to it.)

Given identical data, Occult and Guide produce almost exactly
the same results. (The differences are so small that we can't
really say which program is "right". They're much smaller than
the observational errors.)

-- Bill