怎么计算qt中ftp的上传速度

根据这个信号

这是我现在写的
double timeNow = 0;
timeNow = timer.elapsed();

    m_intervalSent += _used - m_currentSent;

    m_currentSent = _used;



    if(timeNow - m_timeInterval > 300)
    {
        double speed = m_intervalSent * 1000 / (1024 * 1024) / (timeNow - m_timeInterval);

        double leftTime = (_total - _used) /(1024 *1024) / speed;


        m_intervalSent = 0;
        m_timeInterval = timeNow;



    }