Disclosing the basis for estimates is rarely required, but always an excellent practice. If you spell it out up front, it's easier to deal with any customer who may complain about being mislead. Also, it's really handy for internal reviews. Your internal reviewer will save time (spelled "m.o.n.e.y") if s/he doesn't have to go on a treasure hunt for all the data necessary to reproduce and prove the advertising disclosure.
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...gone fishing.