Contrast Index (CI) was designed to pick points that will be useful on paper. So it "kind of" picks Zone I to Zone VIII.

Using the chart from Stephen and choosing 6, 7 and 8 stops, is similar to finding where Zone VII, VIII and IX hits the paper. (Giving CI for N+1, N and N-1).

I double-checked by looking at the graph (which includes a Time/CI curve), and it looks like it might be right.