//precalc

      m_pgf8k_scale=(4.0 /* this is +12dB or so)*0.5;
      m_pgf8k_scale_i = 0.5/m_pgf8k_scale;

      double b=m_pgf8k_scale;
      double tmp = b*b+4*b;
      if (tmp<0.0) tmp=0.0;
      b = (-b+sqrt(tmp)) * m_pgf8k_scale_i;
      if (b<0.0)b=0.0;
      m_pgf8k_actcenter = (int) (pow(b,0.3333333333333) * 1000.0);

      m_slider_center=max(m_pgf8k_actcenter,600); // dont give less than 60% of the fader to negative gains

      m_pgf8k_sc1 = m_pgf8k_actcenter/(double)m_slider_center;
      m_pgf8k_sc2 = (1000-m_pgf8k_actcenter)/(double)(1000-m_slider_center);
      m_pgf8k_sc1i=1.0/m_pgf8k_sc1;
      m_pgf8k_sc2i=1.0/m_pgf8k_sc2;

// x is scale-factor (1.0 = +0dB, etc)

    double b=m_pgf8k_scale;
    double tmp = b*b+4*b*x;
    if (tmp<0.0) tmp=0.0;
    b = (-b+sqrt(tmp)) * m_pgf8k_scale_i;
    if (b<0.0)b=0.0;
    b =  pow(b,0.3333333333333) * 1000.0;

    if (m_pgf8k_actcenter != m_slider_center)
    {
      if (b <= m_pgf8k_actcenter) b *= m_pgf8k_sc1i;
      else b = m_slider_center + m_pgf8k_sc2i*(b-m_pgf8k_actcenter);
    }
    return b; // 0..1000