How to scale Factors and Position for Blend

Question

I'm stuck at this problem for few hours but can't seem to make progress.

I have a perfect annulus (Fig 1), but when I increase the inner ellipse, I'm loosing the drawing (Fig 2)

I guess that I need to scale Blend.Factor and/or Blend.Positions, but I don't know hoy to do it.

This is 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 you.

EDIT: I'm using the code in this question by BlueMonkMN