If going by the ages old belief of adding 10 20 mm to your sit bone measurement really got you the correct size saddle for your butt, then why are so many people having trouble finding a saddle that fits them.

You don't always sit on that one spot of the saddle that is measured for your sit bones and the profiles change drastically from rear to front. So maybe Spesh' does something different in the way they design their saddles and size them. I'd go with their recommendation until you actually know better from experience.