A test item to undershat how this works.

VertSBCalc.pas

{ This event does 2 things: 1) it obtains the initial value of the Bottom Bound
  by checking for a current saved value of zero, and 2) it sets the Bottom Bound
  value if the current saved value is non-zero. The current saved value has been
  set to zero when the form was created, or to some integer value when the user
  has clicked the SBB button. }

procedure TfVertScrollDemo.VertScrollBox1CalcContentBounds(
  Sender: TObject;
  var ContentBounds: TRectF);
begin
  Beep;                                              // to let the user know this routine was entered
  if not(BB > 0) then
    BB := Trunc(VertScrollBox1.ContentBounds.Bottom) // save current value
  else
    ContentBounds.Bottom := BB;                      // cause Bound Bottom value to change

  edHT.Text := VertScrollBox1.Height.ToString;       // display the vertical scroll box height property
end;

Extracted from th VertScrollDemo project.