It would be simpler to enter the "other" values into a separate field in the first place than to try to extract just those that are not part of your defined value list.
If you have FileMaker Advanced, you can set up a custom function that takes the list of values returned by the ValueListItems function and the list of selected values in your check box field and returns only the values that are not one of the predefined values. Do you have FileMaker Advanced? (You can probably find such a function on the Brian Dunning web site of posted custom functions.
I guess you're right about just having a custom field. I was hoping to have it connected to specific categories, but I don't know how many custom entries we'll end up having anyway so I can start with just the one field and see what we need down the line. Too bad there's not an easier way to do it! I did find Dunning's website with the custom functions when I was searching for a solution but I don't have Advanced. Oh well… Thanks again for your advice!
You shouldn't need more than one additional field no matter how many "other" values you want them to be able to select as each "other" value should be in this field but in a different record each time.
You can add a value named "other" to your value list and use a script performed by the OnObjectModify trigger to open a custom dialog where the user enters that "other" value, but into this added field instead.
You might also find some of the enhanced value selection examples in "Adventures in FileMaking #2" to be useful alternatives to your current design.
Oh cool, great idea!