How to measure the pixel width of a digit in a given font / size (C#)

I am trying to calculate the pixel width of Excel columns, as described in this post, using the official formula from the OpenXML specification. However, in order to apply this formula, I need to know the Maximum Digit Width of the Normal font, which is the pixel width of the widest numeric digit. The OpenXML specification gives this example as a clarification:

Using the Calibri font as an example, the maximum digit width of 11 point font size is 7 pixels (at 96 dpi).

I have checked that this is correct by visually examining a Calibri 11-point digit and it is indeed 7 pixels wide. So, I am trying to create a method that will return the maximum digit width for any font / size.

I have followed the recommendations made in this question, but it doesn't produce the results I am expecting.

Here's my test code:

var font = new Font("Calibri", 11.0f, FontStyle.Regular);

for (var i = 0; i < 10; i++)
{
    Debug.WriteLine(TextRenderer.MeasureText(i.ToString(), font));
}

This reports that all digits have a width of 15.

Any suggestions please?

Thanks, Tim