Instead of measuring your seam.....try this. Cut 3 pieces, each 1 1/2" by 4". Sew the long sides together using what you think is a 1/4" seam. Press and measure. Your piece should measure 3 1/2 inches by 4 inches.
If it is less than 3 1/2" you need to take a narrower seam. Hope this helps you.