I bought my Phenom SL from a Specialized Concept Store. They measured my sitbone width, but it's not a direct reading to which saddle is required: they then ask you what type of riding you do. There are three stickman type pictures: 1) time-trialler, clearly tucked and aero; 2) aggressive racing; and 3) Fredelicious with 30mm of spacers and 20mm of seatpost.

Now if you don't fall into any of those, I'm not sure how much 'float' there is in the scale for selecting required saddle width. I consider my position to be slightly less aggressive than if I were racing. Long story short; the 143mm Phenom SL chewed my butt.