The gutter is the space at the interior of the book or bound set of pages. Books with more pages need a wider space for the binding. There is a calculation for this, based on the number of pages in the book or manuscript.
Unless you have a very thick book, you really don't need to do any fancy calculating. I've published a book with only 100 pages and a couple with 300+ pages and used the same gutter on all of them. If you set the inside border at 0.75, you should be ok.
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