C / C++ / MFC

nPage 
      Specifies the page size. A scroll bar uses this value to determine the 
      appropriate size of the proportional scroll box. 

case WM_SIZE: 
 
        // Retrieve the dimensions of the client area. 
 
        yClient = HIWORD (lParam); 
        xClient = LOWORD (lParam); 
 
        // Determine the maximum vertical scrolling position. 
        // The two is added for extra space below the lines 
        // of text. 
 
        yMax = max (0, LINES + 2 - yClient/yChar); 
 
        // Make sure the current vertical scrolling position 
        // does not exceed the maximum. 
 
        yPos = min (yPos, yMax); 
 
        // Adjust the vertical scrolling range and scroll box 
        // position to reflect the new yMax and yPos values. 
 
        si.cbSize = sizeof(si); 
        si.fMask  = SIF_RANGE | SIF_PAGE | SIF_POS; 
        si.nMin   = 0; 
        si.nMax   = yMax; 
        si.nPage  = yClient / yChar; 
        si.nPos   = yPos; 
        SetScrollInfo(hwnd, SB_VERT, &si, TRUE);