Put the carpenters square on the mask so that each end of short side of the square touches the side of the circle. Draw a line along this side and mark the middle of the line. This draws what mathematicians call a bisected chord.
Move the square so that the corner lines up with the middle mark and the short side is on the line you've drawn. Draw a line along the long side. From geometry we know that this line passes through the center of the circle. This effectively mimics a carpenters centering tool. Do this twice and the intersection of the long lines is the center of the circle. I did it multiple times to get an idea of the error in the method (from sloppy measuring etc)
Peter
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