Cordelia,
A few ideas that might help.
On a scrap piece of fabric, sew what you think your 1/4 seam is. Then measure it. You want a scant 1/4 because when you press your seam open, you lose a little on each side. That little bit can compound over time.
If you have some scrap blocks, try this. Cut them to 4 inches. Then w/ a pencil and your handy dandy ruler, mark your 1/4 seams. Sew on the inside of your drawn line. Press and measure and see if you can get a 12.5 inch row.
Clear as mud?