You need to use some creative thought in following the instructions. If you do things exactly as you are instructed, there will not be a precise fit.
You can see this for yourself. Measure the length of a piece of paper, very accurately, and then fold it in half. Now measure the length from the top of the folded sheet to the fold (using a magnifying glass). The new length will be a fraction of a millimetre less then half the length of the unfolded sheet. The fold itself takes up some of the length. This will be even more obvious if you try it with a thick sheet of cardboard.
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