Publisher's typically offer spine width calculators on their websites for self - publishers to use. (99designs.com)
Any other custom book printing options will also need to be final in the quote, as they too many affect spine width and dimensions. (starprintbrokers.com)
This is a crash course on designing book covers for print, including spine width, publisher logo or button, what to put on the back cover, margins and trim, etc.. (creativindie.com)