function TwipsPerPixelX(Canvas : TCanvas) : Extended;
begin
  result :=  1440  / GetDeviceCaps(Canvas.Handle, LOGPIXELSX);
end;

function TwipsPerPixelY(Canvas : TCanvas) : Extended;
begin
  result :=  1440  / GetDeviceCaps(Canvas.Handle, LOGPIXELSY);
end;

procedure TForm1.Button1Click(Sender: TObject);
begin
  ShowMessage(FloatToStr(TwipsPerPixelX(Form1.Canvas)));
  ShowMessage(FloatToStr(TwipsPerPixelY(Form1.Canvas)));
end;

Private Declare Function GetDeviceCaps Lib  "gdi32"  _
      (ByVal hdc As Long, _
      ByVal nIndex As Long) As Long

Private Declare Function GetDC Lib  "user32"  _
      (ByVal hwnd As Long) As Long
      
Private Declare Function ReleaseDC Lib  "user32"  _
      (ByVal hwnd As Long, _
      ByVal hdc As Long) As Long

Private Const LOGPIXELSX =  88 
Private Const LOGPIXELSY =  90 

Private Function TwipsPerPixelX() As Double
Dim hdc As Long
  hdc = GetDC(Application.hWndAccessApp)
  TwipsPerPixelX =  1440  / GetDeviceCaps(hdc, LOGPIXELSX)
  ReleaseDC Application.hWndAccessApp, hdc
End Function

Private Function TwipsPerPixelY() As Double
Dim hdc As Long
  hdc = GetDC(Application.hWndAccessApp)
  TwipsPerPixelY =  1440  / GetDeviceCaps(hdc, LOGPIXELSY)
  ReleaseDC Application.hWndAccessApp, hdc
End Function