I agree with what RHM says. ^^^
Also, please see these pressure graphs. I think these are from Bontrager. As you can see, if you ride in a race position, the sit bone width at the rear is almost irrelevant. I am more or less a "posture 2" rider, and for me the middle section of the saddle matters the most. The pictures make it obvious why.
Borrowed from
this article.
I am absolutely not against measurement, but I don't agree with the presumption that this
one measurement is enough to tell you what saddle is right for you.