Once you have determined the effective film speed for your film and equipment, you need to establish the optimum development time to create a negative that will produce a good range of printable tones. To do this we expose a test length of the entire range from zone l through zone lX. Use the same film but rate it at the effective ISO as determined by the first test, and with the 18% gray card under the same light conditions as before, focus the lens at infinity. Assuming your lens only stops down to f16 and opens to f2.0, a typical exposure series might look like this.
zone l f16@1/125
zone ll f11@1/125
zone lll f8@1/125
zone lV f5.6@1/125
zone V f4.0@1/125
zone Vl f2.8@1/125
zone Vll f2.0@1/125
zone Vlll f2.0@1/60
zone lX f2.0@1/30
Then shoot blank frames until your camera's exposure counter indicates 20 exposures and then repeat this exposure sequence on the remainder of the roll.
In the darkroom remove the film from the cassette, fold the length in half and cut the film into 2 strips. Store the 2nd strip and develop the first strip using the same development parameters used in your previous film speed test. Then print each frame to create reference patches using the same exposure you used in the film speed test to create zone 0 filmbase-fog. [note: your zone l patch should match the density of your chosen zone l patch from the previous test.]
Take note of the density obtained in zone V. Optimum zone V density for a condenser enlarger is going to be somewhere between 0.55 and 0.65 above filmbase-fog, (in a diffusion enlarger 0.65 to 0.75)
If you have access to a densitometer it will simplify the process. You can plot the density curve and easily measure the zone V film exposure to see if it falls into this optimum range. If the density in zone V is in the optimum range for your enlarger, you have found your "normal" development time and the proper ISO speed rating for this film. However, if you find the zone V density to be too high, you can develop the 2nd test strip using a shorter development time (reduce development time by a third) for comparison. If the density is too low, you would increase development (increase by a third) and then fine tune from there. Decreasing or increasing development by one third sounds quite dramatic, but remember, you are primarily modifying the highlight density in your film. The shadow densities are minimally effected by development.
Once you are satisfied that you have your optimum normal development you can shoot a second exposure sequence to test for normal-minus 1 development. Develop that film strip reducing your "normal" development time by one-third. Compare the final printed test patches from that film strip. If the zone Vl patch matches the density of the zone V patch from the normal development you then have your normal-minus 1 development time. For normal-plus 1 you would start by increasing your normal development time by a third and expect to do some fine tuning.
Take careful notes because these tests can be the basis for further refining your workflow. If you change developers or want to explore the qualities of a new film these tests can be a good starting point to ensure consistently good results.
If you shoot 35mm be careful when increasing development ( N+1) times. Increased development produces increased grain, which can be a problem in areas of consistent textureless value such as an open sky.
To sum up, the zone system is a way to gain tighter control of your technical process. It allows you to bring pre-visualized esthetic qualities to life in your prints, even before the exposures are made in the field.