I need to replace my elderly 50 inch umbrella, the problem being that sizes given by shops and manufacturers are misleading, some quote rib length, or closed length or span over the top or beneath? I've seen advertised 8 spoke 50 inch umbrellas which are basically square - does anyone have one, is the span the same as a normal shaped umbrella or is it better or worse?? If you have a square umbrella what is the span across the sides? Thanks.