For ZS users film speed is very simple and I have always used it. Ansel said Z1 is .1 over base fog as you have noticed. Exposure and development for Z1 is the least flexible of all the tones. For some reason box speed usually won't allow you to achieve .1 over base fog. If Ansel had said that zone 1 density should be .05 above base fog then box speed would work and people would just place their tone desires at a higher zone.
Using the Zone system everything becomes relative to what you want your prints to look like. It all becomes subjective. Ansel could just as easily said that Z1 should be .2 above base fog and then everyone would have changed how they place their shadows. I was taught to use .1 over base fog and so I place my shadows up around Z3 depending on the situation. Once you have established your normal development for Z7 or 8 and have that contrast look fixed in your mind then out in the real world with your spot meter you are trying to visualize what you want for contrast. Where you want detail and where you don't.