actually, the drop in the arch is 12" in the video,... mea culpa
Another way to determine the radius of the arch would be to use a little math...
radius = height/2 + width^2/(8*height)
where the "height" is the "drop" of the arch (6" in the video) and the "width^2" is the width of the arch squared(36" in the video). Measure down the centerline of the arch from the arch peak and set your nail or screw. Hook your tape on the nail and mark the arch.
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