I submitted the link to ARCT to a couple of websites, digital-digest.com and http://www.videohelp.com/tools?tool=ARCT, the first comment on videohelp.com has got me thinking, they mention 352x576 which is a format some digital broadcasters use and some standalone DVD recorders use, now I haven't added that resolution to the calculator yet but apart from that 352x2=704 which some capture devices use for the width instead of 720.

If someone could get their thinking cap on and have a muse over this; if you capture letterbox footage at 720x576 and the same footage at 704x576 the pixel width of the actual letterbox image will be the same (if there's sufficient empty space on either side), so which width resolution (704 or 720) should be used to calculate the aspect ratio of the footage?