be where you want it.
if your zipper is slightly longer than the three sides of the book, then you can get your book to open flat. to do this, you will glue the ends of the zipper further in the spine towards the middle of the book so that the zipper can open all the way.
my zipper was the exact size of the three sides of the book. this means i had to choose between opening the book all the way flat or having a larger compartment area with the zipper closer to the edges of the book. i went for the larger compartment, so i stretched my corners out 