You can do it with one of your rulers. I take my 4 1/2 inch ruler, line up the 45* angle line on the left side of your square. You should be able to put the angled side of the ruler on the diagnol of the square, point to point. Draw your line down the center. Then take your ruler and measure out 1/4 inch from the drawn line, 1 side at a time, and draw your sewing line.
Fons & Porter makes a tool for this, but it works this way too.