I don't understand exactly what you're asking, but have you tried using an auto-enter calculation rather than a lookup? That gives you more flexibility, e.g. Boolean operators, etc.
If you could supply an example of the problem (values supplied, values returned, expected values), I might be able to help you with the calculation.
Thanks for the posting in response to my message. Here is the link: http://filemaker.custhelp.com/app/answers/detail/a_id/4741/kw/lookups
I tried to follow its instructions but i could not succeed with the lookup of zip code higher as it was still thinking of doing the lookup for zip code lower.