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The difference between dBd and dBi is 2.15 dB, not 2.51. But essentially, you got the right idea. In general, it's unlikely for people to get steady reception on channels that are below about -15 dB NM. You can certainly reach below that on good tropo days / evenings, but continuous reliable reception at these levels is pretty rare.
Also keep in mind that the NM values are at the center of a bell curve (think statistically). For a fixed transmitter and receiver, the time and space variability will cause the true signal strength to vary a bit. On average, you are expected to get the predicted NM values, but there will inevitably be changes that vary with time of day, seasons, weather, tropospheric effects, ionospheric effects, and other random factors.
Propagation modeling is just a tool. There are lots of things it does not account for, and there's always the possibility of errors in the data. The tools can never be perfect, but they can provide a good first-order approximation of the environment.
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