make another block, but this time, use a mechanical pencil to draw your stitching lines on the back.
When your done with your block and it's 12 1/2", measure your seam allowances to see what your foot really produces and write it on a piece of masking tape and leave on your machine to remind you.
That is a beautiful block, by the way. I love pink and green :thumbup:
Went back and read the other suggestions. they sound easier than mine, especially the one about the three 1.5" strips.