No idea how to measure without the Specialized Assometer.
When I sat on the Assometer, it involved a formula that was printed on the back of it, so it wasn't JUST my measurement, but they also took into account that I ride a tri bike (aero). Different bikes = different sizes. Between my sit bones, their chart with the formula and the type of bike I ride, they came up with 143.
If your LBS is a Specialized dealer, I'd ask when they're getting the Assometer. It's literally a piece of cardboard with a gel on one side, a ruler at the bottom of that and a formula on the other side. Specialized should send it to them overnight -- to help them sell saddles. But that doesn't help you right now.
You could also consider buying the saddle directly from
http://www.specialized.com Check their online return policy, but I believe they have a 30-day no hassle return policy and ship the second item (replacement item) free of charge if you don't like the size/color, etc. of what you just ordered.
BTW, I think I read or heard that 70% of the population fits a 143 Specialized saddle.
Good luck.