I have a problem that has baffled both myself and my photography teacher.

I set up my Nikkormat EL on a tripod and shot a gray card three times on succeeding days--FP4, 36, shot the whole roll. Shooting at indicated meter reading I should have had negatives that printed at Zone 5. The first roll printed at Zone 7—I have already calibrated my in camera meter and it is right on. I shot another roll, same procedure but I cut back development time by 10% i.e. I went from 8.5 (8.5 was indicated on the D-76 bottle) minutes to 7.39. These negatives printed at Zone 7 also, no change. I thought I made a mistake and re-shot and re-developed at 7.39—no change in the print zones! I shot yesterday and cut development time 20% i.e. to 6.42. There is a very, very slight change in the negatives and print but hardly noticeable. What, oh, what could be wrong that my negatives aren’t thinning as a cut back on development time? Here’s the kicker. I followed the same process with my 120 film and when I cut development by 10% my print moved right down from Zone 7 to Zone 5!!! Yet, every step in the processing of the two films is exactly the same.

Following instructions in Brian Lav’s book, Zone System: Step By Step Guide for Photographers I am attempting to calibrate my in camera light meters with my film and find the correct length of time for my film development.

1. I am using Ilford FP4.
2. My cameras are a Nikkormat EL and a Mamiya 645E.
3. I am using D-76 at a one to one ratio at 68 degrees as my developer. My fix is Lauder 763 Rapid Film Fixer with Hardner. I also use HG85, Heico Perma Wash.
4. On step one I found that my in camera meter in my Nikkormat EL was correct but that I had to make a correction to my ASA on my Mamiya . I did the latter.
5. Following step two of Brian’s process I put up a ‘gray card’ (in shade at 12:30 pm) and shot an entire roll of FP4 using the Nikkormat’s indicated meter readings. I should have then had negatives that printed as zone 5. Instead they printed as Zone7—Ok, that would indicate that I needed to cut back on development time. I shot the next day and cut development time by 10% (from 8.5 to 7.39). No change, the negatives were still printing at Zone 7. I cut development time 20% (8.5 to 6.42) and there is just the slightest thinning of my negatives.
6. Checking my Fixer chemicals I put a piece of film leader in the beaker and the film went clear within 30 seconds so I am assuming my fixer is still potent.
7. When developing I agitate on the minute and at 30 seconds.

Can you think what might be wrong?