You won’t be able to finalize your notebook printing or book’s cover artwork until you know how wide the spine will be. Accordingly, the width of the book is determined by the two steps- Number of pages in the book and the thickness of the paper used to create the pages.
To ensure accuracy, it is always best to get the spine width directly from the printer who will be producing your book. So as soon as you have determined the book’s final page count, contact your printer to discuss paper options. Then once you decide on the paper type and thickness, your printer will be able to precisely calculate how wide you need to create the spine on the book’s cover artwork.