You will need to have a table that is a parent to the 'Foo' table. This will allow you to do a Summary field on the Nr field. You can then create a button to Add a New Foo record and within that button, you would pass the Summary value +1 to the Nr field. It sounds like you're trying to create something to generate the next in line to the Nr value (a consecutive number). If that's the case, I would recommend against this particular setup and suggest the possibility of doing a calculation that would be based on the Record ID# instead. This would eliminate any possible errors if records are deleted in the future.
