Different scaling when drawing Metafile on different machines - winforms

Question

I'm having problem to get consistent scaling in my WinForms application that uses a Metafile with millimeter as the unit of measure. I wrote a small sample application to illustrate the problem.
This is how the application looks on a Windows 7 desktop machine:
This is how the application looks on a Windows 8 laptop machine:
The source code:
private void MainForm_Paint(object sender, PaintEventArgs e)
{
var g = e.Graphics;
var blueBrush = new SolidBrush(Color.Blue);
var bluePen = new Pen(blueBrush);
g.DrawRectangle(bluePen, 0, 0, 200, 200);
g.DrawLine(bluePen, 100, 0, 100, 200);
g.DrawLine(bluePen, 0, 100, 200, 100);
g.DrawString(g.DpiX+" dpi", new Font("Arial", 10), blueBrush, 0, 205);
Metafile metafile;
var size = new Size(200, 200);
using (var stream = new MemoryStream())
{
using (Graphics offScreenBufferGraphics = Graphics.FromHwndInternal(IntPtr.Zero))
{
IntPtr deviceContextHandle = offScreenBufferGraphics.GetHdc();
metafile = new Metafile(stream, deviceContextHandle, new RectangleF(0, 0, size.Width, size.Height), MetafileFrameUnit.Millimeter, EmfType.EmfPlusOnly);
offScreenBufferGraphics.ReleaseHdc();
using (Graphics mg = Graphics.FromImage(metafile))
{
mg.PageUnit = GraphicsUnit.Millimeter;
var redPen = new Pen(new SolidBrush(Color.Red));
const float scaleFactor = 0.75f;
mg.ScaleTransform(scaleFactor, scaleFactor);
mg.DrawLine(redPen, 0, 0, 200, 200);
mg.DrawLine(redPen, 0, 200, 200, 0);
}
}
}
g.DrawImage(metafile, 0, 0, 200, 200);
}
Both machines are set on 96dpi, yet the Win8 machine renders the metafile (the red cross) smaller.
The scale factor 0.75 is calculated from the difference between the standard 72 dpi and the current 96 dpi, 72/96=0.75, is this correct? Edit: See answer below why this will not work.
But mostly, why is it scaled differently on the Win8 machine and what setting can I fetch to compensate? Seems like the Win8 machine needs a scale factor around 1.25 to make the red cross align with the blue rectangle.
Thanks!

Answer 1

Found the answer myself after reading this code project article. Turns out that the screen size on the machine (1920x1200 on desktop and 1600x900 on laptop) affect the resolution of the metafile. The assumption of 72dpi that was used to calculate the scale factor 0.75 was somewhat correct on my desktop machine, but not on the Win8 laptop.
The metafile resolution can be fetched from the metafile header, and then used to calculate the correct scaling factor:
var metafileHeader = metafile.GetMetafileHeader();
float sx = metafileHeader.DpiX/g.DpiX;
float sy = metafileHeader.DpiY/g.DpiY;
mg.ScaleTransform(sx, sy);
The complete code can be found here.
Then I get correct scaling on both machines: