I'm trying (failing) to create a button template, to be shared between libraries. The template is (currently) basically a bordered circle inside a bordered circle. In order to make the inner border's size smaller than the outer, I use a Converter on the binding. I'd like to pass in a property of the TemplatedParent as a ConverterParameter but it just doesn't work. Here's my Style def (in a ResourceDictionary):
Code:
    <SolidColorBrush x:Key="MyBorderFillColour">Yellow</SolidColorBrush>
    <SolidColorBrush x:Key="MyBorderEdgeColour">#ff652f00</SolidColorBrush>
    <SolidColorBrush x:Key="MyGeneralFillColour">#ffffffbd</SolidColorBrush>
    <s:Int32 x:Key="MyBorderThickness">10</s:Int32>

    <l:RelativeSizeConverter x:Key="RelativeSizeConverter" />

    <Style x:Key="MyButtonStyle" TargetType="Button">
        <Setter Property="Template">
            <Setter.Value>
                <ControlTemplate TargetType="{x:Type Button}">
                    <Grid>
                        <Ellipse 
                            Fill="{StaticResource MyBorderFillColour}" 
                            StrokeThickness="2" 
                            Stroke="{StaticResource MyBorderEdgeColour}" 
                            Height="{TemplateBinding Height}" 
                            Width="{TemplateBinding Width}" />
                        <Ellipse StrokeThickness="2" 
                            Stroke="{StaticResource MyBorderEdgeColour}" 
                            Fill="{StaticResource MyGeneralFillColour}"
                            Height="{Binding Path=Height,
                                RelativeSource={RelativeSource TemplatedParent}, 
                                Converter={StaticResource RelativeSizeConverter},
                                ConverterParameter={StaticResource MyBorderThickness}}"
                            Width="{Binding Path=Width,
                                RelativeSource={RelativeSource TemplatedParent}, 
                                Converter={StaticResource RelativeSizeConverter},
                                ConverterParameter={Binding Path=BorderThickness,RelativeSource={RelativeSource TemplatedParent}}}" />
                        <TextBlock 
                            Text="{TemplateBinding Content}" 
                            HorizontalAlignment="Center" 
                            VerticalAlignment="Center" 
                            Foreground="Black" 
                            FontFamily="Calibri"
                            FontWeight="Bold"
                            FontSize="17" />
                    </Grid>
                </ControlTemplate>
            </Setter.Value>
        </Setter>
    </Style>
The second Ellipse's Height works using a StaticResource, but Width doesn't work using the TemplatedParent approach.

Any idea how to achieve my aim?!?

Thanks for any help,
Toot