I believe it's very important to understand the theory. The solution to your problem is retesting, and you shouldn't be so concerned about the film speed. As to exactly what N, N+1, etc should be in terms of aim CIs and why is another interesting point of theory.
On another note, I am finding all the contributions on this thread very informative, thank you everyone. I am not sure, however, as to the best way to interpret my results to find the development times. I realise that one can take great pictures without any of this, but I would love to know my personal timings for N-1, N, and N+1. Perhaps I need to contact the tablet for the curves, and to perform a separate EI test in natural light using a non-uniform target and my camera.