You're right, it is only a theory. I have settled for less setback if I cannot get it to my "ideal" measurement on frames with steep seat tube angles, which happens to be the case for my frame size. My go to seat post is the Velo Orange Grand Cru Long Setback seat post, which allows for a rather generous 30.2mm of setback. I used this in combination with vintage saddles like Turbos or Rolls, but it still fell short about 10mm with the saddle pushed all the way back. I have ridden as is for a while and it felt just fine. Recently, I found a solution by using more modern saddles like the Selle Italia SLR, which allows for more setback thanks to the longer rails. It feels like the additional 10mm of setback allows for a more smoother pedal stroke and I do feel my upper hamstrings being used more than before. That being said, it is still a work in progress trying to figure out what my body likes and does not like, and how this adjustment translates to power/speed.