Mitch--
          Let me clarify my statement. The point that I was making was that
  for any given calculation, microstrip/stripline/embedded, etc, the VALUE
  for the dielectric constant MUST be the same VALUE. Which formula that
  you use, SHOULD NOT have different values for the same material. That 
  number, as stated by several others, may vary due to many things, but the
  dielectric constant value, as determined by the end product use, used in the 
  formula should remain the same! You agree?
                                                    Bob Vanech