http://u.qdnx.org/file/rozzin-20131231T192838-db3rpx5.html
Oh, the last "72" in "(N/72)**2 * 72" is the inherent size of the input SVG in points, unless the SVG's inherent size is in pixels, in which case #ImageMagick does the completely different thing that you'd actually expect. !TIL