As Peckish said, we talk about a 1/4" seam, but you don't measure the seam, you measure the resultant patch. Keep moving your needle or tape or whatever you use for a guide until the patch measures the correct finished width. Also, this can be a moving target. Your fabric and thread choices can affect it. It's best to double check at the beginning of a project and make any adjustments. One other thing to check is that you are not pressing a tiny pleat into the seam with your iron.