How did you come up with the 38" measurement? Is that "finished" or does it include the seam allowances? If you measured it by measuring the fabric, are you sure that you didn't stretch the fabric (especially if it was on the bias) during the measurement? Tim's method calculates the sides of the triangle that will complete your corners if your measurement is unfinished, but I'm not sure that it correctly takes into account the seam allowances. At any rate, please cut a triangle out of PAPER first and make sure it is the right size to be SEWN to your quilt top (realizing that there will be seam allowances on where the paper and the quilt top meet) before you start doing the same with your fabric.