Firemonkey: Shrink text font to fit in TLabel

I am attempting to lower the font size of a TLabel if its text is to large to fit in the confines of the label. I didn't see any properties I could set on the label to achieve this, so I have tried writing my own method. My method works by using TCanvas.TextWidth to measure the width of the text in a label, and shrink the font until the width of the text fits within the width of the label.

void __fastcall ShrinkFontToFitLabel( TCanvas * Canvas, TLabel * Label )
{
    float NewFontSize = Label->Font->Size;

    Canvas->Font->Family = Label->Font->Family;
    Canvas->Font->Size = NewFontSize;

    while( Canvas->TextWidth( Label->Text ) > Label->Width && NewFontSize > MinimumFontSize )
    {
        NewFontSize -= FontSizeDecrement;
        Canvas->Font->Size = NewFontSize;
    }

    Label->Font->Size = NewFontSize;
}

This works some of the time, however other times it does not shrink the font near enough. It seems as if the value I get from calling Canvas->TextWidth is a lot of times, much smaller than the number of pixels wide the label actually needs to be in order to fit the text.

Am I using Canvas->TextWidth incorrectly? Is there a better way to calculate the width of a string, or to re-size the font of a TLabel so its text fits within its demensions?

Edit:
In this case, I am passing in to my function, the TCanvas that my label is sitting in. I have tried using that TCanvas as well as Label->Canvas. Both give me the same number for text width, and both are short of the actual value in pixels needed to display the whole string.

The following code is taken from code that works in an FMX application, modified slightly to remove arrays that are being iterated through and declaring a variable locally to the function. It is being run in a TForm method. Canvas here is the Form's Canvas. You can see that I'm using "- 35" at one point - this might be because the numbers weren't quite right.

double InitialFontSize = 30;    
Canvas->Font->Size = InitialFontSize;
StoryHeadlineLabel->Font->Size = InitialFontSize;
bool fits = false;
do
{
    double widthA = Canvas->TextWidth (StoryHeadlineLabel->Text);
    if (widthA > StoryHeadlineLabel->Width - 35)
    {
        StoryHeadlineLabel->Font->Size --;
        Canvas->Font->Size --;
    }
    else
        fits = true;
    if (StoryHeadlineLabel->Font->Size < 6)
        fits = true;
} while (!fits);