Fair question, and I see now the six decimal points on the title of this thread, while empirically derived, are less useful than a moving average.

I don't think the argument is that your nine significant figure number is "less useful" but that it is far too precise considering how it was derived. I'm not even sure your five significant figure numbers are appropriately precise; they are based on data that is 500 years old and/or extrapolated into the future.

Remember that you should not claim a level of precision for your calculation that is greater than the level of precision of the data being used. If you can show that x significant figures is appropriate for the least precise data point, then your result should not exceed x significant figures.

I'd suggest three significant figures is appropriate given the source of the data points.

So, the SSB for your first period is 179; the SSB for your second period is 179.

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