So the Specialized chart says that you are in, let's call it, Category 2, with recommended saddle widths 143 to 155. You are not in the group for which 130 mm is recommended. Don't bother looking at the upper and lower scales, they are not you.

Whether you go for a 143 or a 155 is based on how aggressive your seating position is. You can guage that by the height difference between your saddle and the top of your handlebars. If they're even, you are probably in the middle of the scale, and should try a 143. If your bars are higher than 2 or 3 cm above the saddle, you could try the 155 first. If your bars are a few cm below the saddle, you should try, again, the 143.

Specialized saddles are relatively flat but with some arch. Notice the dimension of the 143 is at least 1 cm wider than your sit bones on each side. If you are looking at a saddle that is a little more arched than a Specialized, look for a saddle a little wider than the Spec. If the saddle is a little flatter than the Specialized (such as a lot of WTB's), look for a little narrower. But in this by guess and by golly guestimating, YMMV. Also try to become aware of whether the middle of the saddle is wide or narrow. It matters, too.

You're not going to find an algorithm for saddle scaling. You are being shown some reasonable guidance for choosing a reasonable starting point.

You could also repeat the process sitting on the Ass O Meter, leaned forward to approximate your usual riding position. That might get you closer to what looks real.