Different saddles of the same supposed width will fit differently. It's way more complicated than get the width right and the saddle will fit. You just have to ride them and see if they work. No amount of measurement will tell you that in advance. That said, it doesn't hurt to start with a saddle that's theoretically the right width. Doesn't mean it'll work, though.

Also leather saddles work for some people in a particular range of position, but they don't work for all people or for any person without regard to position. Composite saddles are interesting in that they can be made in any shape, just like people.