When cutting your fabric, it helps to place the line for the measurement you want ontop of the fabric, not off the edge of the fabric. If you are placing that line off the edge of the fabric, you are cutting the piece a "scant" too small. There should not be extra fabric showing beyond the line, however. Just make sure the entire thickness of the line is ontop of the fabric, nothing more, nothing less. This "scant" stuff is something we all struggle with to get right. You have tons of company!