Оформил: DeeCo
Автор: http://www.swissdelphicenter.ch

// 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;