Jorge O.,
It would seem, at first glance, that what you suggest would work. I have found in my experience that it will not be accurate. The reason is that Zone I, Zone II, and approaching Zone III all fall on the toe of the film's characteristic curve. Typically, these tonalities will also fall on the shoulder of the paper's characteristic curve. Therefore the negative densities and corresponding print tonalities are not well separated there.
I have heard that most humans can visually distinguish a film density change of .07 and above. Therefore if one has no access to a densitometer, I would rely on a visual inspection of the film before I would rely on the tonal difference in the print that you suggest. I think that this would be more accurate.