code: @Override
      public double calculateScaleForBondLength(double modelBondLength) {
          if (Double.isNaN(modelBondLength) || modelBondLength == 0) {
              return rendererModel.getParameter(Scale.class).getDefault();
          } else {
              return this.rendererModel.getParameter(BondLength.class).getValue() / modelBondLength;
          }
      }
docs: Given a bond length for a model, calculate the scale that will transform
  this length to the on screen bond length in RendererModel.
  
  @param  modelBondLength the wanted model bond length in screen length
  @return returns the scale that causes the drawn bond lengths in pixels to match
  the given model bond length