Although is not the answer to your question,I don't think is a good idea to use an automatic deletion or, at least, such a small value: it has happened several times that an update introduced some issues and you won't be able to take a real advantage of a scheduled backup if you set it this way.
After all, they don't use so much space and deleting the by the red key is very quick, in case.
At least, I'd try and set a value of about 30 but this is not a guarantee of "capturing" also a previous version if no updates are released in this amount of time.