addition in binding Path

Your best bet would be to use a converter something like this :

[ValueConversion(typeof(double), typeof(double))]
    class AddWidthConverter:IValueConverter
    {
        public object Convert(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
        {
            double currentValue = (double)value;
            return currentValue + 70;
        }

        public object ConvertBack(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
        {
            throw new NotImplementedException();
        }
    }

Then declare your converter in your Window.Resources and then use it in your xaml like this :

Width="{Binding ElementName=txtTest, Path=ActualWidth, Converter={StaticResource AddWidth}