Also, the shape of the saddle affects the effective width quite a bit. If it slopes to the outside, then the area that's available to your sitbones is quite a bit narrower.

What I did was to measure my sitbones by placing a piece of paper on top of a Pilates mat on a hard surface. Then I circled the indentations so I had a permanent butt-print that I could bring with me saddle shopping. By setting the paper on the saddle, I could get a pretty good idea of whether a saddle was going to support my sitbones or not. You could make different prints for aero position vs. more upright.