Is there a way to determine in millimeter or in twips the
position of the pointed character versus the size of the
richtext box itself ?

Best regards
Jean-Claude

Drop a textbox on the form to run  the code un-alterd..
    TextBox1
And of Course, a RichTextBox
    RichText1

Code is adapted form the Hyperlink example at
    http://www.vb-world.net/demos/hyperlink/CHyperlinks.cls.html

D.

Option Explicit

Private Declare Function SendMessage _
    Lib "user32" Alias "SendMessageA" ( _
    ByVal hwnd As Long, _
    ByVal wMsg As Long, _
    ByVal wParam As Long, _
    lParam As Any) As Long

Private Const EM_GETLINE = &HC4
Private Const EM_LINEFROMCHAR = &HC9
Private Const EM_LINEINDEX = &HBB
Private Const MAXLINELENGTH = 512
Public Sub CalcSpot()

Dim strLine As String
Dim lngLine As Long
Dim lngLineIndex As Long
Dim lngSelStart As Long
Dim lngHWnd As Long
Dim bLine(MAXLINELENGTH - 1) As Byte
Dim i As Integer, iPos As Integer
Dim WhichMode As Integer

    lngHWnd = RichTextBox1.hwnd
    lngSelStart = RichTextBox1.SelStart
    'Get the line number we're on
    lngLine = SendMessage(lngHWnd, EM_LINEFROMCHAR, lngSelStart, 0&)
    lngLineIndex = SendMessage(lngHWnd, EM_LINEINDEX, lngLine, 0&)
    lngSelStart = lngSelStart - lngLineIndex

    bLine(0) = MAXLINELENGTH And 255
    bLine(1) = MAXLINELENGTH \ 256
    'get the line and save it in the byte array
    SendMessage lngHWnd, EM_GETLINE, lngLine, bLine(0)
    For i = 0 To MAXLINELENGTH - 1
      If bLine(i) = 0 Then
        Exit For
      End If
      strLine = strLine & Chr$(bLine(i))
    Next

    WhichMode = vbMillimeters
    'WhichMode = vbInches
    Text1.Text = ScaleX(TextWidth(Mid$(strLine, 1, lngSelStart)),
Me.ScaleMode, WhichMode)

End Sub

Private Sub RichTextBox1_SelChange()
    CalcSpot
End Sub