You have to stop with the pocket at this point and measure out the width needed for the insert. Why? Because I said so. Make it JUUUUUST under the total width of the pocket to make it snug but not a pain in the bum to remove and insert.
huh huh huh... "insert"
Same goes for the height.
You also want to round off the corners with your scissors for these purposes as well. And also to protect the fabric from getting a hole poked in it or something.