Dynamic Shapes on Chart

New guy here, so please go easy on me. I'm working on a project that could use two perpendicular lines to follow the cursor around on a chart. I've gotten it to work to a point, but my problem lies in what I believe to be the scaling of the PlotArea. I define the start/end of both lines as the boundaries of the PlotArea, but it is evident that they don't start directly on the boundaries. Also, as the cursor moves down and to the right, the discrepancy between the cursor and the intersection point of the perpendicular lines becomes larger. What I want to find out is how to make the two lines perfectly "in sync" with the cursor and the PlotArea boundaries. The conceptual workbook is attached in case anyone would want to take a look. Thanks in advance!

My code is placed in the Chart's sheet.

Option Explicit
Public YPosition, XPosition As Variant

Sub Test()

Dim HorizontalLine, VerticalLine As Shape
Dim StartX, StartY, EndX, EndY As Double

With ActiveChart
    StartX = .PlotArea.Left + .Axes(xlValue).Width
    StartY = .PlotArea.Height + .Axes(xlCategory).Width
    EndX = .PlotArea.Width
    EndY = .PlotArea.Top
End With

If ActiveChart.Shapes.Count > 1 Then
    Do Until ActiveChart.Shapes.Count = 0
        ActiveChart.Shapes(1).Delete
    Loop
End If
If HorizontalLine = "" Then
    With ActiveSheet
        Set HorizontalLine = .Shapes.AddLine(StartX, YPosition, EndX, YPosition)
        Set VerticalLine = .Shapes.AddLine(XPosition, StartY, XPosition, EndY)
    End With
End If

End Sub

Sub Chart_MouseMove(ByVal Button As Long, ByVal Shift As Long, ByVal X As Long, ByVal Y As Long)

Dim ElementID As Long
Dim Arg1 As Long
Dim Arg2 As Long

Me.GetChartElement X, Y, ElementID, Arg1, Arg2
XPosition = X
YPosition = Y

Call Test

End Sub

The shape x and y values are on a different scale to the mouse x and y values.

try adding a fudge factor

Me.GetChartElement x, y, ElementID, Arg1, Arg2
XPosition = x * 0.74
YPosition = y * 0.74

Call Test

But it's more complicated than that as zoom setting has an affect.