Environmental health inspectors, checking on commercial premises (such as restaurants), work to a guide temperature of -18C, or below, for food storage. So your freezer temperature is probably 'about right' unless you're using it to freeze food, rather than simply storing food which is already frozen. In which case you might want to set it to be slightly colder, as a temperature of around -23C is often cited as being best for freezing food in a domestic appliance.
Chris