Max...
          We routinely produce Mil-Std pwb's with characteristic impedence 
  values of up to 100 ohms, relative to the reference plane. I assume that 
  when you refer to the "width of the dielectric", you mean the dielectric 
  separation between the signal layer and its reference plane. We obtain
  our values with a separation of approx. 10 mils and a trace width of
  .0065 +/- .0015 mils. However, we are using polyimide which has a better
  dielectric constant. I may not have totally understood your question, so
  maybe my answer does't help. However, I can't believe that this is all
  black magic and you have a very difficult design problem. You may also
  want to discuss the issues with another supplier.  If you would like to
  discuss this further, please feel free to  contact me. Hope it helps.
                                                    Regards,
  Bob Vanech                                                  Bob
  Norden Systems
  (203) 852-4810

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