Select only negative numbered query results

Hello, just hoping for a bit of help on this one:

I have built a query which is linked via an ODBC database connection. I have
added 3 linked tables to my query. In my query I have a field called "NET
AVAIL". This field is a calculated field based upon the following expression:

Field Input is NET AVAIL: [ON HAND] - [REQRD]
Table Input is -- Blank --
Total Input is Expression

The way I have this written now it querys and displays all of my records
accurately, but it displays all my numbers which result in a positive "NET
AVAIL" as well as a negative. I would like to filter this calculation to
display on the results of this calculation which are negative.

I've tried many, many different things and just can't get it.
Any help is greatly appreciated! Thanks.