Rotation maths not working correctly in WPF

Question

I'm using Adorner class to create a rotation thumb for my controls. Here's the relevant part of the class:

Private initialAngle As Double
Private startVector As Vector
Private currentPoint, centerPoint As Point
Private rotateTransform As RotateTransform

Private RotateThumb As Thumb

Public Sub New(adornedElement As UIElement)
  MyBase.New(adornedElement)
  AddHandler RotateThumb.DragStarted, AddressOf HandleRotateStart
  AddHandler RotateThumb.DragDelta, AddressOf HandleRotate
End Sub

Private Sub HandleRotateStart(sender As Object, e As DragStartedEventArgs)
  Dim AE As FrameworkElement = TryCast(Me.AdornedElement, FrameworkElement)
  centerPoint = New Point(AE.Width / 2, AE.Height / 2)

  Dim startPoint As Point = Mouse.GetPosition(AE.Parent)
  startVector = Point.Subtract(centerPoint, startPoint)

  rotateTransform = TryCast(AE.RenderTransform, RotateTransform)
  If rotateTransform Is Nothing Then
    AE.RenderTransform = New RotateTransform(0, centerPoint.X, centerPoint.Y)
    initialAngle = 0
  Else
    initialAngle = rotateTransform.Angle
  End If
End Sub

Private Sub HandleRotate(sender As Object, e As DragDeltaEventArgs)
  Dim AE As FrameworkElement = TryCast(Me.AdornedElement, FrameworkElement)

  currentPoint = Mouse.GetPosition(AE.Parent)
  Dim deltaVector As Vector = Point.Subtract(centerPoint, currentPoint)

  Dim angle As Double = Vector.AngleBetween(deltaVector, startVector)

  Dim myrotateTransform = TryCast(AE.RenderTransform, RotateTransform)
  myrotateTransform.Angle = initialAngle + Math.Round(angle, 0)
  AE.InvalidateMeasure()
End Sub

Protected Overrides Sub OnRender(drawingContext As DrawingContext)
  MyBase.OnRender(drawingContext)

  If Mouse.LeftButton = MouseButtonState.Pressed Then
    drawingContext.DrawEllipse(New SolidColorBrush(Color.FromArgb(100, 0, 255, 0)), New Pen(Brushes.Green, 1), centerPoint, 10, 10)
    drawingContext.DrawEllipse(New SolidColorBrush(Color.FromArgb(100, 255, 0, 0)), New Pen(Brushes.Red, 1), currentPoint, 7, 7)
  End If
End Sub

While this does (kind of) rotate the element, the problem is that currentPoint does not appear to be where my mouse pointer is (the red ellipse of OnRender doesn't follow mouse correctly). I suspect this might have something to with RenderTransform that I apply on my element affecting DrawingContext. Or is it about the mathematics I'm doing here?