<Style x:Key="Autorize_Button" TargetType="Button">
        <Setter Property="Background" Value="{ThemeResource SystemControlBackgroundBaseLowBrush}"/>
        <Setter Property="Foreground" Value="{ThemeResource SystemControlForegroundBaseHighBrush}"/>
        <Setter Property="BorderBrush" Value="{ThemeResource SystemControlForegroundTransparentBrush}"/>
        <Setter Property="BorderThickness" Value="{ThemeResource ButtonBorderThemeThickness}"/>
        <Setter Property="Padding" Value="8,4,8,4"/>
        <Setter Property="HorizontalAlignment" Value="Left"/>
        <Setter Property="VerticalAlignment" Value="Center"/>
        <Setter Property="FontFamily" Value="{ThemeResource ContentControlThemeFontFamily}"/>
        <Setter Property="FontWeight" Value="Normal"/>
        <Setter Property="FontSize" Value="{ThemeResource ControlContentThemeFontSize}"/>
        <Setter Property="UseSystemFocusVisuals" Value="True"/>
        <Setter Property="Template">
            <Setter.Value>
                <ControlTemplate TargetType="Button">
                    <Grid x:Name="RootGrid" Background="{TemplateBinding Background}">
                        <ContentPresenter x:Name="ContentPresenter" AutomationProperties.AccessibilityView="Raw" BorderBrush="{TemplateBinding BorderBrush}" BorderThickness="{TemplateBinding BorderThickness}" ContentTemplate="{TemplateBinding ContentTemplate}" ContentTransitions="{TemplateBinding ContentTransitions}" Content="{TemplateBinding Content}" HorizontalContentAlignment="{TemplateBinding HorizontalContentAlignment}" Padding="{TemplateBinding Padding}" VerticalContentAlignment="{TemplateBinding VerticalContentAlignment}"/>
                    <Rectangle Name="Line" Height="1.5" Width="{TemplateBinding ActualWidth}" Fill="White" Opacity="0" VerticalAlignment="Bottom" />
                    </Grid>
                </ControlTemplate>
            </Setter.Value>
        </Setter>
    </Style>

<Rectangle Name="Line" Height="1.5"
           Width="{Binding ActualWidth,
                           ElementName=Root,
                           Converter={StaticResource OneFourthConverter}}"
           ... />

class OneFourthConverter : IValueConverter
{
    public object Convert(object value, Type targetType, object p, string language) =>
        (double)value / 4;

    public object ConvertBack(object value, Type targetType, object p, string language) =>
        (double)value * 4;
}

<Page.Resources>
    <local:OneFourthConverter x:Key="OneFourthConverter"/>
</Page.Resources>