WPF Circle Rotate via MouseDown MouseMove and MouseUp, and compute rotated angle via Vector

<Window x:Class="WpfApp158.MainWindow"
        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:local="clr-namespace:WpfApp158"
        mc:Ignorable="d"
        Title="MainWindow" Height="450" Width="800">
    <Window.Resources>
        <local:SizeConverter x:Key="sizeConverter"/>
    </Window.Resources>
    <Grid>
        <Ellipse x:Name="elp"
              Width="{Binding ActualHeight,RelativeSource={RelativeSource Mode=FindAncestor,AncestorType={x:Type Window}},
              Converter={StaticResource sizeConverter}}"
              Height="{Binding ActualHeight,RelativeSource={RelativeSource Mode=FindAncestor,AncestorType={x:Type Window}},
              Converter={StaticResource sizeConverter}}"
              MouseDown="elp_MouseDown"
              MouseMove="elp_MouseMove"
              MouseUp="elp_MouseUp"
              RenderTransformOrigin="0.5,0.5"
              >
            <Ellipse.RenderTransform>
                <RotateTransform x:Name="rotateTransform"/>
            </Ellipse.RenderTransform>
            <Ellipse.Fill>
                <LinearGradientBrush StartPoint="0,0"
                                     EndPoint="0,1">
                    <GradientStop Color="Orange" Offset="0.0"/>
                    <GradientStop Color="Orange" Offset="0.2"/>
                    <GradientStop Color="Yellow" Offset="0.2"/>
                    <GradientStop Color="Yellow" Offset="0.4"/>
                    <GradientStop Color="Green" Offset="0.4"/>
                    <GradientStop Color="Green" Offset="0.499"/>
                    <GradientStop Color="Red" Offset="0.499"/>
                    <GradientStop Color="Red" Offset="0.501"/>
                    <GradientStop Color="Green" Offset="0.501"/>
                    <GradientStop Color="Green" Offset="0.6"/>
                    <GradientStop Color="Cyan" Offset="0.6"/>
                    <GradientStop Color="Cyan" Offset="0.8"/>
                    <GradientStop Color="Blue" Offset="0.8"/>
                    <GradientStop Color="Blue" Offset="1.0"/>
                </LinearGradientBrush>
            </Ellipse.Fill>
        </Ellipse>
    </Grid>
</Window>



//cs
using System.Text;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Data;
using System.Windows.Documents;
using System.Windows.Input;
using System.Windows.Media;
using System.Windows.Media.Imaging;
using System.Windows.Navigation;
using System.Windows.Shapes;
using System.Diagnostics;
using System.Globalization;

namespace WpfApp158
{
    /// <summary>
    /// Interaction logic for MainWindow.xaml
    /// </summary>
    public partial class MainWindow : Window
    {
        private Point prevPt { get; set; }
        private Point originPt { get; set; }
        public MainWindow()
        {
            InitializeComponent();
        }

        protected override void OnRenderSizeChanged(SizeChangedInfo sizeInfo)
        {           
            base.OnRenderSizeChanged(sizeInfo);
            Point tempPt = new Point(elp.ActualWidth / 2, elp.ActualHeight / 2);
            originPt = elp.TranslatePoint(tempPt, this);
            Debug.WriteLine($"{originPt.X},{originPt.Y}");
        }

        

        private void elp_MouseDown(object sender, MouseButtonEventArgs e)
        {
            if(e.LeftButton==MouseButtonState.Pressed)
            {
                prevPt = e.GetPosition(this);
                elp.CaptureMouse();
            }
        }

        private void elp_MouseMove(object sender, MouseEventArgs e)
        {
            if(e.LeftButton==MouseButtonState.Pressed)
            {
                Vector prevVec=Point.Subtract(prevPt, originPt);
                Point pt=e.GetPosition(this);
                Vector nowVec=Point.Subtract(pt, originPt);
                double deltaAngle= Vector.AngleBetween(prevVec, nowVec);
                Console.WriteLine($"deltaAngle:{deltaAngle}");                 
                rotateTransform.Angle += deltaAngle;
                this.Title=Math.Round(rotateTransform.Angle,2).ToString();
                prevPt = pt;
                Debug.WriteLine(rotateTransform.Angle.ToString());
            }
        }

        private void elp_MouseUp(object sender, MouseButtonEventArgs e)
        {
            if( e.LeftButton==MouseButtonState.Released)
            {
                elp.ReleaseMouseCapture();
            }
        }
    }

    public class SizeConverter : IValueConverter
    {
        public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
        {
            double dValue = 0.0;
            if(double.TryParse(value?.ToString(), out dValue))
            {
                return dValue - 100;
            }
            return dValue;
        }

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