// Getting a TScrollbar control to Show a proportional thumb:
procedure TForm1.Button1Click(Sender: TObject);
 var
  info: TScrollInfo;
 begin
  FillChar(info, SizeOf(info), 0);
  with info do
  begin
  cbsize := SizeOf(info);
  fmask := SIF_PAGE;
  nPage := ScrollBar1.LargeChange;
  end;
  SetScrollInfo(ScrollBar1.Handle, SB_CTL, info, True);
 end;
 // Same for a TStringGrid
procedure TForm1.Button1Click(Sender: TObject);
 var
  info: TScrollInfo;
 begin
  FillChar(info, SizeOf(info), 0);
  with info do
  begin
  cbsize := SizeOf(info);
  fmask := SIF_ALL;
  GetScrollInfo(StringGrid1.Handle, SB_VERT, info);
  fmask := fmask or SIF_PAGE;
  nPage := 5 * (nmax - nmin) div StringGrid1.RowCount;
  // whatever number of cells you consider a "page"
 end;
  SetScrollInfo(StringGrid1.Handle, SB_VERT, info, True);
 end;