Using the designers method of placing one fabric on top of the other, drawing a line, then sewing 1/4 inch on either side of the line, cutting apart creates two half square triangles. Rule of thumb is to add 7/8 inches to the finished block size. So for finished 2 1/2 inch block, add 7/8 inch = 3 1/4 inch cut block. My guess is that the designer is adding a little extra to allow for squaring up the block or assumes a generous 1/4" seam line. . Using the same method to make quarter square triangles draw 'X' then sew 1/4 inch on either sides of the lines. Cut into four blocks. To the size of the finished block, add 1 1/4 inches. So you would need to start with 3 3/4 inch block. For 3" finished. Start with 3 7/8 (3" + 7/8") block for half square triangle and 4 1/4 (3" + 1 1/4") inch block for quarter square triangle. Hope this helps. Reason for using these methods is so the bias edges are not on the outside of the block.