It's not as much about your perfect 1/4" seam allowance as it is about the ending result. If you cut correctly and sew a full 1/4", your are going to be too small for your end result. I was told to cut 2 strips, 2" x 4". Sew them together on the 4 inch side and iron the seam to the side. Do you have a strip that is 3 1/2" x 4"? If you don't have 3 1/2 inches then you need to sew a scant quarter, which is one or two threads less than your original seam. Hope this helps.