I don't draw the corner to corner diagonal. I draw the 2 sewing lines. Line up the 1/4" marking on your ruler corner to corner, then draw the line. Turn your square 180 and draw the other one. I found that even though you have to draw twice as many lines, the result is worth it to me! There are several rulers on the market that are made for this, just a half inch wide.
http://generationqmagazine.com/2011/...iangle-part-2/