Your existing blocks will become 2 sides of a right angle triangle. To make half square triangles that will finish at 12", so add 7/8"with 12" finished sides, or about 13". The outside edges will be on the bias, so starch a lot, or make them quarter square triangles. To do quarter square triangles, you need the size of the third side of the triangle is 12 x sqrt(2), or 12 x 1.41 = 17.0" finished square to make quarter = square triangle. Then add the seam allowance (1.25" for a quarter square triangle), so 18.25".