To measure 1/4 inch, you can put your needle down and use your metal measuring guide against the needle, put masking tape down perpendicular to the needle, and use it for a guide. A scant 1/4 inch means 1 thread or two less that the 1/4 inch. When you press the seam to the dark fabric, it seems to use up a little of the measurement. There should be some tutorials about pinning the seams so that they end up matching exactly.