This is not something I created, but it's definitely something I use.  There's a great article here about why an adding an element of the unknown is necessary in planning.  Averages are great, but by definition they only get it right half of the time.  I recommend that when you have an important decision to make, get the known ranges and plug them into the spreadsheet below.  You'll have a much better idea on how things will end up, statistically speaking of course.

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