Inserting Image into DocX using OpenXML and setting the size

The sizes, in EMUs (English Metric Unit — read this for a good explanation), are set in the Extents (the Cx and Cy).
In order to get a pic into a DocX I usually do it like so:

1. Get the image’s dimensions and resolution
2. Compute the image’s width in EMUs: wEmu = imgWidthPixels / imgHorizontalDpi * emuPerInch
3. Compute the image’s height in EMUs: hEmu = imgHeightPixels / imgVerticalDpi * emuPerInch
4. Compute the max page width in EMUs (I’ve found that if the image is too wide, it won’t show)

5. If the image’s width in EMUs is greater than the max page width’s, I scale the image’s width and height so that the width of image is equal to that of the page (when I say page, I’m referring to the “usable” page, that is minus the margins):

   var ratio = hEmu / wEmu;
wEmu = maxPageWidthEmu;
hEmu = wEmu * ratio;

6. I then use the width as the value of Cx and height as the value of Cy, both in the DW.Extent and in the A.Extents (of the A.Transform2D of the PIC.ShapeProperties).

Note that the emuPerInch value is 914400.
I have this running (in a service) but I don’t have the code with me right now.

UPDATE

This is the code I use:

var img = new BitmapImage(new Uri(fileName, UriKind.RelativeOrAbsolute));
var widthPx = img.PixelWidth;
var heightPx = img.PixelHeight;
var horzRezDpi = img.DpiX;
var vertRezDpi = img.DpiY;
const int emusPerInch = 914400;
const int emusPerCm = 360000;
var widthEmus = (long)(widthPx / horzRezDpi * emusPerInch);
var heightEmus = (long)(heightPx / vertRezDpi * emusPerInch);
var maxWidthEmus = (long)(maxWidthCm * emusPerCm);
if (widthEmus > maxWidthEmus) {
  var ratio = (heightEmus * 1.0m) / widthEmus;
  widthEmus = maxWidthEmus;
  heightEmus = (long)(widthEmus * ratio);
}

In my case, my page’s measures are in cm, hence the emusPerCm you see above.

UPDATE 2 (to answer @andw)

To have the file locked during the minimum possible time, open it with a FileStream and pass the resulting stream to the StreamSource property of the BitmapImage:

var img = new BitmapImage();
using (var fs = new FileStream(fileName, FileMode.Open, FileAccess.Read, FileShare.Read)) {
    img.BeginInit();
    img.StreamSource = fs;
    img.EndInit();
}
// The file is now unlocked
var widthPx = img.PixelWidth;
...