Не нашел хорошего решения, но нашел приемлемое:
int top = Application.Top + Application.PageSetup.TopMargin + Application.PageSetup.HeaderMargin + Application.Commandbars["Ribbon"].Height;
int left = Application.Left + Application.PageSetup.LeftMargin;
Form popup = new Form{ Top = top, Left = left, StartPosition = FormStartPosition.Manual, Width=400, Height=300};
popup.Show();