You would plot the values on a graph with densities on the Y-axis and the zones on the X-axis. Yes, you could do this test for each development time tested and you could plot each curve in a different color on the same graph. This is called the "characteristic curve" of the film. The flat part on the left near Zone I is the "toe," and the flat part on the right is the "shoulder," and the middle is the straight line portion. Tones that fall in the straight line portion will have good separation, while tones that fall on the toe will be muddy (with respect to each other), and tones that fall on the shoulder will be "blown out."

In the process of plotting these curves, you might discover that one development time gives you a normal contrast range, and another gives you N+1, and maybe another is N-0.5, etc.

Say for example that you've targeted Zone VIII density at 1.2, and you get that density at Zone VIII with your developer of choice at 10 min. Now what if you also did a test at 12 min., and discover that Zone VII has a density of 1.2 at that development time? That would be a one-zone expansion (N+1), and you could use that increased development time to get more contrast in a flat scene. You could also compare the 12 min. curve to the 10 min. curve and discover how much of a change you get at Zone V when you get a one zone increase at Zone VII, which is one of the questions we started out with. Maybe then you might try a test at 15 min. to see if you can get an expansion of N+2 (Zone VI with a density of 1.2), or you might discover that the film doesn't have enough density for an expansion of N+2.

Ultimately, of course the print will tell you all you need to know, but the process of plotting the graphs will teach you a lot about how film works, how to make it do what you want, and what it can't do easily.