C# – Non-scaling Pen in GDI+

I need to create a pen which draws with a width of 0.5 mm regardless of the scale of the graphics transformation matrix. Here is what I am doing:

Note: The sample code is C++/CLI but answers using C# are welcome.

// assume that g is a Graphics object
int dpi = g->DpiX;
float dotsPerHalfInch = dpi * 0.5;

// to extract scale factor from the transformation matrix...
// create two points distant one unit apart...
array<PointF> ^points = gcnew array<PointF>{
    PointF(0.0f, 0.0f), PointF(1.0f, 0.0f)
};

// transform them...
g->Transform->TransformPoints(points);

// get distance after transformation...
float dX = points[1].X - points[0].X;
float dY = points[1].Y - points[0].Y;
float distance = Math::Sqrt(dX * dX + dY * dY);

// reverse the effects of any scale factor in graphics transform
float penWidth = dotsPerHalfInch / distance;

Pen ^halfInchPen = gcnew Pen(Color::Black, penWidth);

Doesn't seem to work on screen (96 dpi) …gives me a pixel wide pen. On the PDF printer (600 dpi) it gives me a pen far more thicker than half an inch.

What am I doing wrong? What is the correct way to create a non-scaling pen in GDI?