Updating data field if less than another value

I'm implementing some smart forms that pull in a value for employee count on an organization. I'm able to take the custom smart form field and write it into our standard SFDC if the field is already blank. What I'm hoping to do is write the value in if it is either blank or less than the value in the custom smart form field.

If I use a less than = token for the custom field, I get an error that the token is invalid. Any ideas on how I can set up the smart list?