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AL: Thanks for the  advice. The precision of the premises cannot really justify the precision of the 8.625 result (3 decimal places). I have reduced it to a precision of 1 decimal place (8.6). That is, in accord with standard mathematical ""tolerance error"" principles, I believe the precision of a multiplication cannot be better than the least precision of the factors. I have also added the word ""approximately"". This is because the rule of thumb factor of 2.5 is admitted to be ""roughly"" accurate in the source. (Its actually 2.543 but that is only implied not stated).   "