WPF Converter with binding 1/2

WPFConverterBinding1_2.cs

public class ZIndexConverter : DependencyObject, IValueConverter
    {
        public string Size
        {
            get { return (string)GetValue(SizeProperty); }
            set { SetValue(SizeProperty, value); }
        }

        public static readonly DependencyProperty SizeProperty =
            DependencyProperty.Register(nameof(Size), typeof(string), typeof(ZIndexConverter), new PropertyMetadata("0,0",OnSizeChanged));

        private static void OnSizeChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
        {
            if (d is ZIndexConverter instance)
            {
                instance.UpdateHeight();
            }
        }

        private double height = 0;

        private void UpdateHeight()
        {
            if (Size.Contains(',') && Size.Split(',') is string[] values && values.Length == 2)
            {
                bool px = double.TryParse(values[0], out double nx);
                bool py = double.TryParse(values[1], out double ny);

                if (py)
                {
                    height = ny;
                }
            }
            else
            {
                height = 0;
            }
        }

        public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
        {
            if(value is string zindex)
            {
                bool ok = double.TryParse(zindex, out double newz);
                if(ok)
                {
                    return height - newz;
                }
            }

            return height;
        }

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