Converter

  public class LastColumnConverter: MarkupExtension, IValueConverter
  {
      public override object ProvideValue(IServiceProvider serviceProvider)
      {
          return this;
      }

      public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
      {
          var width=value as double?;
          double.TryParse(parameter?.ToString(), out var max);
          if (width.HasValue && max>0)
          {
              if(width< max)
              {
                  return new GridLength(0);
              }
              else
              {
                  return new GridLength( width.Value-max);
              }
          }

          return 0;
      }

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