Any Specialized dealer should have the Body Geometry Fit System that actually measures your personal sit-bones width:



. . . and they have varying widths of BG saddles to accomodate you and your riding style:





That's the only quantifiable means I know to get close - - the rest is (literally) seat-of-the-pants evaluations. But maybe it will help. Personally I can't stand the Body Jihad line; but that is just me; YMMV.