I see target points above fbf as largely meaningless. It all depends on the actual curve shape and how you want the shadows to separate, based on your personal expectations and printing values. One reason that folks often rate film lower than box speed is to push the shadow futher up toward the straight line section of the curve. And in this respect, so-called staraight line films will dig way further down than something with a long toe. I really visualize the curve when I shoot, and only use the Zone System as a shorthand labeling tool to assign which batch of development I want a particular neg to go into (N verus N+1 etc). With some films I can easily get shadow separation way down to Zone 0. Only
with something like Pan F would I use Zone III as the metered shadow value. I work with a lot of different kinds of film and almost never botch an exposure unless the lighting very suddenly changes.