Change the Position of the Modal form at runtime

Question

There is a modal form in my Delphi 2007 application. I already have applied MinHeight,MaxHeight, MinWidth and MaxWidth constraints on the form.

If the screen resolution is below the Min/Max constraints, I want to re-size and re-position the form according to the screen resolution.

I have written the following code in OnCreate event handler function to handle the screen resolution.

procedure TfrmMyForm.FormCreate(Sender: TObject);
var      
  WorkArea: TRect;
  iTitleHeight: Integer;
begin
  inherited;
  //-------------------------------------------------------------------
  //Adjust Height/Width of the form if min/max values 
  //don't fall under the current resolution.
  iTitleHeight := GetSystemMetrics(SM_CYCAPTION); //Height of titlebar
  WorkArea := Screen.WorkAreaRect;

  if(Self.Constraints.MinWidth > WorkArea.BottomRight.X) then
  begin
    if(Self.Constraints.MaxWidth > WorkArea.BottomRight.X) then
    begin
      Self.Constraints.MinWidth := WorkArea.BottomRight.X;
      Self.Constraints.MaxWidth := WorkArea.BottomRight.X;
      Self.Position := poDesigned;
      SetBounds(0,0,WorkArea.BottomRight.X, WorkArea.BottomRight.Y - 5);
    end
    else
    begin
      Self.Constraints.MinWidth := WorkArea.BottomRight.X;
    end;
  end;
  if(Self.Constraints.MinHeight > WorkArea.BottomRight.Y) then
  begin
    if(Self.Constraints.MaxHeight > WorkArea.BottomRight.Y) then
    begin
      Self.Constraints.MinHeight := WorkArea.BottomRight.Y - iTitleHeight;
      Self.Constraints.MaxHeight := WorkArea.BottomRight.Y;
      Self.Position := poDesigned;
      SetBounds(0,0,WorkArea.BottomRight.X, WorkArea.BottomRight.Y -  5);
    end
    else
    begin
      Self.Constraints.MinHeight := WorkArea.BottomRight.Y;
    end;
  end;
//-------------------------------------------------------------------
end;

At design time I've set the Position property as follows

Position := poCentreScreen

Now there are three problems I am facing when the screen resolution is low like 1024x768 and Min & Max Height/Width constraint values are above this resolution.

I am changing the Position property to poDesigned because otherwise the form doesn't move to the position where I want it to be.

When I run my application on system have dual monitors it behaves unexpectedly.

I have also added a custom menu item in the system menu of the form and when I change the Position property to poDesigned in OnCreate function it resets the system menu and removes the custom menu item.

Is there any way to re-size and re-position the form without modifying the Position property?

Thanks in anticipation.