Every cutting size in a pieced pattern assumes a 1/4 in seam allowance. A 2.5 in cut square finishes at 2 in because a quarter inch disappears into the seam on each side. If your seam is 5/16 in, each square finishes at 1-7/8 in and a 20-square row comes out 2.5 in short. Borders will not fit and points will be cut off.
Checking your seam
Cut three strips 1.5 in wide and about 6 in long. Sew them together side by side and press. The centre strip should measure exactly 1 in wide. If it is narrower, your seam is too wide; if wider, too narrow. Adjust the needle position or move the guide and test again.
The scant quarter inch
Pressing the seam to one side takes up a thread or two of fabric. Many quilters sew a scant 1/4 in, a thread's width less than a full quarter, so the pressed block measures true. Whether you need it depends on your machine, thread and fabric; the three-strip test tells you.
Pressing
Press seams toward the darker fabric where you can, or open where many seams meet. Press, do not iron: set the iron down, lift it, move it. Sliding stretches bias edges, especially on half-square triangles.
When blocks come out small
Measure a finished block before cutting sashing or borders. If all your blocks are the same slightly-small size, cut sashing and borders to the real measurement and the quilt still goes together flat; the finished size just shrinks a little. Patterns on this site list the block size so you have a number to check against.