You have no control over the file names. The file names are always generated from the metadata fields, and then shortened to a maximum total path length in order to work across filesystems and OSes.
If you absolutely, positively must have control over the filenames, then calibre is unfortunately not for you.
Keep in mind that you needn't look at the filenames at all -- complex metadata queries are usually much more useful.
The official statement:
http://manual.calibre-ebook.com/faq....tory-structure