How to scale factors and position for blending

I have been stuck with this issue for hours but cannot make any progress.

I have a perfect ring (Fig. 1), but when I enlarge the inner ellipse, I lose the drawing (Fig. 2)

I am guessing that I need to scale the Blend.Factor and / or Blend.Positions, but I don't know how.

These are my Factor and Position arrays:

_Factors = new float[] { 0f, 1f, 0f, 0f };
_Positions = new float[] { 0f, 0.25f, 0.5f, 1f };

      

        
        
        
      

    

And this is my Paint function:

   using (Brush oInnerBrush = new SolidBrush(_InnerColor))
     {
        using (Pen oBaseBrush = new Pen(_BaseColor, 2))
        {
           using (GraphicsPath oGraphPath = new GraphicsPath())
           {
              // Outside
              oGraphPath.AddEllipse(oRect);

              // Inside
              oGraphPath.AddEllipse(oRect.Shrink(_InnerWidth));

              using (PathGradientBrush oGradBrush = new PathGradientBrush(oGraphPath))
              {
                 Blend oBlend = new Blend();
                 oBlend.Factors = _Factors;
                 oBlend.Positions = _Positions;
                 oGradBrush.Blend = oBlend;

                 oGradBrush.CenterColor = ((SolidBrush)oInnerBrush).Color;
                 oGradBrush.SurroundColors = new Color[] { oBaseBrush.Color };

                 Area.FillPath(oGradBrush, oGraphPath);
              }
           }
        }
      }

      

        
        
        
      

    

Any ideas? Thank.

EDIT: I am using the code from this BlueMonkMN question