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The graph is a nice way of doing it, but logically the line you plot should always extrapolate to zero tidal stream for zero coefficient.

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I am not sure a zero coefficient exists, at least the SHOM does not contemplate it... saying that the highest and lowest coefficients are 120 and 20 means that the highest amplitude can be 6 times higher than the lowest amplitude.
I don't know if this is the case but it seems to be the assumption on shom tidal calculations.
Maybe it is derived from their harmonic model ?

Agree absolutely that it is much of a theoretical matter