It should be noted that the “factor of two” proposed above has been selected somewhat
arbitrarily based on the authors’ experience, and that it is the philosophy of the approach,
rather than the actual definition of the “grey area” range. Some case studies have been
examined using this approach and the factor of two appears to be appropriate. However, as
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this methodology is implemented, the factor of two may be modified to fit with further case
study information as it becomes available.