A corner gauge is a handy tool for any cross stitcher. Simply lay the gauge on the corner of your fabric to determine where you should start stitching while leaving enough of a margin for framing or finishing.
Using a corner gauge accomplishes a couple of things. It lets you start stitching from the corner of your pattern (instead of from the center), it helps you ensure your stitching will be properly placed on the fabric, and it helps reduce fabric waste.
This vintage-inspired wood corner gauge made by Retromantic Fripperies measures 3 inches square, and has starting points at 2, 2-1/2, and 3 inches depending on how much allowance you need for framing.