MFC: Checkable GroupBox problem

[dpreznik]

Dear experts,

In my application, I am using a custom control that is a GroupBox and a CheckBox instead of a common caption. I got the control here:
http://www.codeguru.com/cpp/cpp/cpp_mfc/article.php/c4149
I had to change the code to allow the checkbox to have multiline text:

CRect rc;
      GetWindowRect(rc);      // Get the rect of the GroupBox
      this->ScreenToClient(rc);
// Change rc to be used for the checkbox
      rc.left += 5;
      rc.right = min(rc.left + czText.cx, rc.right - 5);
      
      int iGroupBoxWidth = rc.right - rc.left;
      double iLinesCount = ceil((double)czText.cx / iGroupBoxWidth);
      rc.top -= (long)((iLinesCount - 1) * czText.cy / 2);
      rc.bottom = (long)(rc.top + czText.cy * iLinesCount);

      if(style == BS_AUTOCHECKBOX)
      {
            m_TitleBox.Create(strText, BS_AUTOCHECKBOX | BS_MULTILINE | WS_CHILD  | WS_TABSTOP, rc, this, ID_TITLE);
      }

As you can see, I change rc.top to have the middle of the checkbox neatly on the same level as the top edge of the groupbox, as in case of single-line caption. The multiline caption should be spread equally over and under that level. I can see that the checkbox is in the right place, so the top of the caption is calculated correctly, but the upper part of the caption text is cut off. I can see the caption not higher than the upper edge of the checkbox. Could you please tell me why this happens?

Thanks.
Here I attached an image to show how it looks.