Graphics.MeasureCharacterRanges Giving Wrong Size Calculations In C#.Net?

I'm trying to render some text into a specific part of an image in a Web Forms app. The text will be user entered, so I want to vary the font size to make sure it fits within the bounding box.

I have code that was doing this fine on my proof-of-concept implementation, but I'm now trying it against the assets from the designer, which are larger, and I'm getting some odd results.

I'm running the size calculation as follows:

StringFormat fmt = new StringFormat();
fmt.Alignment = StringAlignment.Center;
fmt.LineAlignment = StringAlignment.Near;
fmt.FormatFlags = StringFormatFlags.NoClip;
fmt.Trimming = StringTrimming.None;

[Code]....

Now I have two problems.

Firstly, the text sometimes insists on wrapping by inserting a line-break within a word, when it should just fail to fit and cause the while loop to decrement again. I can't see why it is that Graphics.MeasureCharacterRanges thinks that this fits within the box when it shouldn't be word-wrapping within a word. This behaviour is exhibited irrespective of the character set used (I get it in Latin alphabet words, as well as other parts of the Unicode range, like Cyrillic, Greek, Georgian and Armenian). Is there some setting I should be using to force Graphics.MeasureCharacterRanges only to be word-wrapping at whitespace characters (or hyphens)? This first problem is the same as post 2499067.

Secondly, in scaling up to the new image and font size, Graphics.MeasureCharacterRanges is giving me heights that are wildly off. The RectangleF I am drawing within corresponds to a visually apparent area of the image, so I can easily see when the text is being decremented more than is necessary. Yet when I pass it some text, the GetBounds call is giving me a height that is almost double what it's actually taking.

Using trial and error to set the __minimumSize to force an exit from the while loop, I can see that 24pt text fits within the bounding box, yet Graphics.MeasureCharacterRanges is reporting that the height of that text, once rendered to the image, is 122px (when the bounding box is 64px tall and it fits within that box). Indeed, without forcing the matter, the while loop iterates to 18pt, at which point Graphics.MeasureCharacterRanges returns a value that fits.

The trace log excerpt is as follows:

Decrementing font size to 24, as size is 193×122 and limit is 212×64
Decrementing font size to 23, as size is 191×117 and limit is 212×64
Decrementing font size to 22, as size is 200×75 and limit is 212×64
Decrementing font size to 21, as size is 192×71 and limit is 212×64
Decrementing font size to 20, as size is 198×68 and limit is 212×64
Decrementing font size to 19, as size is 185×65 and limit is 212×64
Writing VENNEGOOR of HESSELINK in DIN-Black at 18pt, size is 178×61 and limit is 212×64

So why is Graphics.MeasureCharacterRanges giving me a wrong result? I could understand it being, say, the line height of the font if the loop stopped around 21pt (which would visually fit, if I screenshot the results and measure it in Paint.Net), but it's going far further than it should be doing because, frankly, it's returning the wrong damn results.