Firemonkey: Shrink text font to fit in TLabel - c++

Question

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.

Answer 1

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);