Length of text on a text Path is unpredictable

[martindevans]

I'm trying to get some text to fit onto a certain length path by scaling the font down until the text is small enough to fit into the available space. For plain text this is pretty trivial:

function CreateScaledText(txt, availableSpace)
{
    var fontSize = 19;
    var ruler = new Kinetic.Text({
        text: txt,
        fontSize: fontSize,
        fontFamily: "Calibri",
        fill: 'white',
    });

    while (ruler.getTextWidth() > availableSpace)
        ruler.setFontSize(--fontSize);
}

This approach doesn't work at all for curved text though. If I use the above method to create some text, and then use the calculated fontSize for a textPath of a known length, the text gets cut off! Here is a jsfiddle demonstrating the problem