I went to a bike shop that carries a lot of Specialized saddles - and they have a squishy ruler device - you sit on it (lean forward, help isolate those butt bones) and then you stand up, and then use the measurements on the ruler to identify where the center of your sit bones hit. It measures from zero in the middle, mms to the outside.

So, I adding up 60mm on one side, 80 on the other (apparently not centered, it's hard to do) for an estimate of 140mm - therefore span. I tried the paper-on-hard-surface method at home and came up with 150mm, pretty close.