How to achieve this via 'save to disk' in Calibre?

[JLC42]

When I save a book to my device I want the save template to output the filename as such:

Author - Title (Book X)

Example: Stephen King - The Two Dead Girls (Book 1)

I currently have the following template:

{authors} - {title:re(:, - )}

I want to add a field so that if a book is in a series (only if it's part of a series) will add the paratheses followed by the word book and then the actual number book.

If I do:

{authors} - {title:re(:, - )} (Book {series_index:>3s})

The filename would output as:

Stephen King - The Two Dead Girls (Book 1)

But if I save to disk a book that isn't in a series I would get the following:

Stephen King - Billy Summers (Book )

How can I get the template to put "(Book X)" only if the book is listed in a series?

[JLC42]

Found I can do it via this format (changed the filename structure a bit):

{series}/{series_index:0>2s| [Book |] - }{authors} - {title:re(:, - )}

Output: The Green Mile/[Book 01] - Stephen King - The Two Dead Girls

[JLC42]

I have "{series}/" at the start to create a folder for books that are in a series however the name will be "Green Mile, The", anyone know how I can get the folder to be "The Green Mile".