Octagon measured across the flat sides (not the points)

I need an octagon. I found a 3rd party stencil that does this, but it measures from point to point, and not flat to flat. It is also orientated so there are no flats parallel to the page edges. Is there a stencil that is dimensioned across the flat sides? Also I tried just drawing a line around the existing octagon (after rotating 22.5 degrees so the flats are horizontal and vertical) using magnets at all the corners, but when I draw the line it doesn’t go to the magnets. I must be doing something wrong?
Thanks in advance, Greg