Wpf custom control template - relative font size - wpf

Question

I am creating a custom WPF control that let's say for simplicity sake has a vertical stack panel with a "title" TextBlock, followed by a ContentPresenter. I want the font size for the "title" to be 5 Points LARGER than the size used in the content, which is inherited by whatever container the user places this control in.
How can I specify a font size in the control template for the header element using a relative value without exposing a property like "TitleFontSize" to the user? I want do "add 5".
I tried using a ScaleTransform on the header text block with mixed results (the text block scaled fine but the orientation was modified - I had the text right-justified and it moved "off the control" area when scaled). Also, I am not sure if scale transform would be approprite here.

Answer 1

A more generic way
Value converter
public class MathConverter : IValueConverter
{
public object Convert( object value, Type targetType, object parameter, CultureInfo culture )
{
return (double)value + double.Parse( parameter.ToString() );
}
public object ConvertBack( object value, Type targetType, object parameter, CultureInfo culture )
{
return null;
}
}
Converter Resource
<my:MathConverter x:Key="MathConverter" />
XAML
<TextBlock FontSize="{Binding
RelativeSource={RelativeSource AncestorType={x:Type Window}},
Path=FontSize,
Converter={StaticResource MathConverter},
ConverterParameter=2}" />

Answer 2

I did it with an IValueConverter as follows:
Created a class FontSizeConverter that derives from IValueConverter. The Convert method adds 10 to the value, and the ConvertBack method subtracts 10.
public class FontSizeConverter : IValueConverter
{
#region IValueConverter Members
public object Convert(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
{
return (double)value + 12.0;
}
public object ConvertBack(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
{
return (double)value - 12.0;
}
#endregion
}
Next, I declaried an instance of this class in the XAML template for the control:
<Style.Resources>
<local:FontSizeConverter x:Key="fontSizeConverter"/>
</Style.Resources>
And Finnaly, the FontSize binding uses this converter applied to the inherited FontSize property:
<TextBlock FontSize="{Binding RelativeSource={RelativeSource TemplatedParent}, Path=FontSize, Converter={StaticResource fontSizeConverter}}"
Grid.Row="0" Text="{Binding RelativeSource={RelativeSource TemplatedParent},
Path=Date.Day}" HorizontalAlignment="Right" VerticalAlignment="Top" Padding="2" Margin="2" >
</TextBlock>
This works. But I still do not know if this is the correct answer. Let me know if there is a better way, or if this is appropriate.