Figure out your outside sitbone distance. For example, my center to center is 125, but my outside is ~ 150-160. By Specialized saddle guidelines taking only c t c into consideration, the 143 should be fine. It's not. It's too narrow. I need their 155 to accomodate my outside distance. Ideally, the width should be about the same as your outside measurement.
In fact, I think 150 would be perfect for me, but they don't make it.
Also, a person that has a center to center of 125, but an outside distance of 135-140 would narrower saddle than I.
So, the charts (such as the one published by Specialized) that give a range are leaving out this little bit of info. Though center to center is most important, people with a more narrow or wider outside distance could end up on the wrong size saddle.
Specialized Saddle Guidelines. This is good to read for any saddle brand, but they don't mention outside distance. They just cover it by giving a too wide range for some people. I've noticed a few people on this forum that this chart is not good enough for. Still a great source of info, just incomplete.
Thread mentioning this.