If you're new to it (like I was recently!) you've probably used the report wizard and based your report off either a table or a query.
If you've chosen a query (and probably also used the QBE wizard to build the query) - this is an easy place to add the criteria.
When looking at the query, you'll see a row labeled 'Criteria' below the one that says 'Show' with a checkbox. You can specify either like or where criteria based on the fields you're looking at.
Assuming that your 100% complete criteria means that the values 1-100 (whole numbers) are in a given field, you'd put this into the box below the field you wish to compare to.
<100
In the query itself (SQL view if you go looking) it becomes:
SELECT tbl.fld1, tbl.fld2, tbl.fld3
FROM tbl
WHERE (tbl.fld1 < 100);
Alternatively, you can set the reports recordsource to a SQL string (the above SELECT) command.
Mike