Available/Unused numbers within rnge

[Deman]

Apologize if this has already been covered but I've been looking for a while now.

All I'm trying to do is to create a value list based off a global field of available unused numbers within a set range.

I have unique serial numbers for each record but this is a housing rentals db and they are assigned key numbers per property. These key numbers are within a set range so I'm looking for the easiest way to pull up a list of numbers that are not currently being used. I created separate tables with say 1-200 auto entered serial numbers to compare my key number field to but I'm not exactly sure of the next step as far as the wording of the calc etc. If I can get list of unused numbers into a global field I'm assuming I can assign that to a value list and use it as a drop down.

Any help is appreciated. Thanks.

Using FM Pro 7