I need to draw kind of parallel line based on the original one like this.

I use this code

 const int shift = 10;
 PointF[] clonedPoints = new PointF[polygon.Count];
 float dx = polygon[1].X - polygon[0].X;
 float dy = polygon[1].Y - polygon[0].Y;

 for (int i = 0; i < polygon.Count; i++)
 {
     var k = (float)Math.Atan2(dx / 100, dy / 100);
     float x = polygon[i].X + k + shift;  
     float y = polygon[i].Y + k + shift;
     clonedPoints[i] = new PointF(x, y);
 }

And I get lines like this.

The blue is the cloned one.

So I tried to play with some koefficient k and shift but it does not help.