Don't take me too seriously, I'm poking fun at Bruce's enthusiasm as he makes his point that more exposure is a good thing.
When I look at my graphs, the straight line starts near my speed point (0.1 above Base+Fog).
Roughly mark the graph at 0.1 as Zone I. Mark a Zone on the graph for every other step wedge reading.
Suppose I follow Bruce and meter a dark part of my scene and place it on Zone IV. (Traditional Zone System would put on II - textured shadow). Now in my scene there would be a Zone darker (Traditional Zone I - featureless black). And a Zone darker still (Traditional Zone 0).
What I meant by my tongue in cheek comment was that Traditional Zone 0 (something two stops below my metered shadow) is going to be on the straight line portion.