I went down this road just a few months ago and decided that measuring my sit-bone didn't matter.
I sat on aluminum foil on a carpeted floor and then measured the deepest dents in the foil to get my sit-bone width. I then proceeded to order a crazy expensive SMP saddle that supposedly designed for my sit-bone width. However, after getting the saddle, I found it crazy uncomfortable after over 6 rides and sent it back.
For me, following
Fizik's method for measuring the proper saddle type worked better. They suggest saddle sizes based on your flexibility (or lack there of) which probably determines how rotated your hips are and thus influences the shape of your seat (or at least I think that's the theory). I have very little flexibility and never did, so the Fizik Aliente is the saddle of choice for me. It's relatively wide... wider than I was lead to believe I needed based on the "sit-bone measurement".
I also learned that seats with cut-outs in the middle, or even just indentations in the middle, result in more pressure being put on the seat bone. I have an older Fizik Aliente saddle with no channel and a newer one that does have a channel, and the newer one with the channel results in more pressure on my sit bones. While the cut-outs/channels/whatever might help the soft tissue, they make don't necessarily make the ride more comfortable.