constructor TControl.Create(AOwner: TComponent);
begin
  ...
  FCurrentPPI := 96;
end;

FCurrentPPI := 96;

  TControl = class(TComponent)
  protected
    ...
    FCurrentPPI: Integer;
    ...
  end;

constructor TControl.Create(AOwner: TComponent);
begin
  ...
  FCurrentPPI := 96;
end;

procedure TControl.ScaleForPPI(NewPPI: Integer);
begin
  if not FIScaling and (NewPPI > 0) then
  begin
    FIScaling := True;
    try
      if FCurrentPPI = 0 then
        FCurrentPPI := GetDesignDpi;

      if NewPPI <> FCurrentPPI then
        ChangeScale(NewPPI, FCurrentPPI, True);
    finally
      FIScaling := False;
    end;
  end;
end;

function TControl.GetDesignDpi: Integer;
var
  LForm: TCustomForm;
begin
  LForm := GetParentForm(Self);
                                                                                         
  if (LForm <> nil) and (LForm is TForm) then
    Result := TForm(LForm).PixelsPerInch
  else
    Result := 96;                                                              
end;