I just wanted to thank everyone for taking time from their busy day to
answer my query.  I appreciate all the input.
  It looks like the "final answer" is to spec impedance & tolerance, trace
width and tolerance and spec a minimum layer thickness.  This will allow
different fabricators to use their favorite lams & prepregs, and our
drawings will be in spec no matter who makes the boards.

  Again, thanks for your help!  It is great to have some data to backup your
proposal when you go to your manager to make a change.

    Regards,
        Brandon Luther
        Dataram