ASP.NET

<UserControl x:Class="TextBlockLayoutTest.MainPage"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
    xmlns:converters="clr-namespace:ConvertersSilverlightClassLibrary;assembly=ConvertersSilverlightClassLibrary"
    mc:Ignorable="d" x:Name="testUC">
    <UserControl.Resources>
        <converters:PercentageConverter x:Key="PercentageConverter"/>
    </UserControl.Resources>    
    <Grid x:Name="grid">
        <Grid.ColumnDefinitions>
            <ColumnDefinition x:Name="gridCol1" Width="*" />
        </Grid.ColumnDefinitions>
        <Grid.RowDefinitions>
            <RowDefinition Height="*" />            
        </Grid.RowDefinitions>
        <StackPanel Orientation="Vertical" Grid.Row="0" Grid.Column="0" 
                    MaxWidth="{Binding ElementName=grid, Path=ActualWidth}">       
            <TextBlock TextWrapping="Wrap" >
                test test test test test test test test test test test test test test test test test test test test test test test test test test test test test test test test test test test test
            </TextBlock>        
        </StackPanel>
    </Grid>
</UserControl>

public class PercentageConverter : IValueConverter 
    {
        public object Convert(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture) 
        {
            double d = System.Convert.ToDouble(value);
            d = System.Convert.ToDouble(parameter);
            return System.Convert.ToDouble(value) * System.Convert.ToDouble(parameter); 
            //return 200;
        }      
        public object ConvertBack(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture) 
        { 
            throw new NotImplementedException(); 
        } 
}