this might not sound a big deal but when I was first making quilts like this, the Civil War ones, 5 1/2 blocks and it would never come to size. My teacher told me to use Aurifil. Its only 2 ply and having one less strand makes a difference. Plus it has lower lint. for these I'd probably treat them like half square triangles, increase size a little so that you can match the seams and the square the blocks to the right size.