Henry, I'm a little surprised your Zone 1 density is the same for N-1, N, N+1 rating the film at EI 250 in all cases. TMY is a straight line film with a short toe so Zone 1 tends to move as development times are altered. Just a thought.
I do not have a densitometer. I work with the Analyser from RH Designs. Usually, I take different measures of the density in the same negative photo, and I get measures of 0.09 – 0.11 -0.12 – 0.10 for Zone I. They go in this order. For this reason, I say 0.1 is the density in Z1.