In C++ VS 2010 (MFC), how do I find the starting column of a scrolled CListBox table?

The attached procedure finds where a user selects a column in a CListBox table.

For example, if there is a table that is 20 columns wide, and 10 fit on the screen, then the screen shows | C0 | C1 | C2 | C3 | C4 | C5 | C6 | C7| C8 | C9 |. If the user mouse clicks on the 5th screen column, the procedure returns 5, and the software knows it's column C5.

However, us the user scrolls over 10 columns, the screen shows  | C10 | C11 | C12 | C13 | C14 | C15 | C16 | C17| C18 | C19 | . If the mouse clicks on the 5th screen column (the 15th actual column, C15), the procedure still returns 5, when it's supposed to return 15. How do I add that the user has scrolled right 10 columns, so that the procedure can return 15?
--------------------------------

int CMyListCtrl::GetCurColumn()
{
   // Get the current mouse location and convert it to client
   // coordinates.
    DWORD pos = GetMessagePos();
    CPoint ptPos(LOWORD(pos), HIWORD(pos));
    ScreenToClient(&ptPos);


   long lWide=0;
   long lWideLast;


   
   // Step across to find the column the mouse is in
   for (int nSubIndex=0; nSubIndex<100; nSubIndex++) {


      lWideLast = lWide;
      lWide += GetColumnWidth(nSubIndex);


      if ( lWide>=ptPos.x ) {
         rectHit.left = lWideLast;
         rectHit.right = lWide;
         return( nSubIndex );   // could be break
      }


   } // end for


   return( -1 );


} // end GetSubItemClicked