Bob, I think it should be 6-1/2", not 3-1/4", unless I don't quite understand correctly.
In other words, put marks 6-1/2" out from the corner on both edges. Take your square and draw lines from the marks perpendicular to the respective edge into the board. Where the lines intersect, put your compass, set it to 6-1/2" and draw the radius.
In yet other words: draw a 6-1/2" square at the corner in question. The corner of the square that's farthest away from the corner of the board is the one you put your compass at.