What is the mathematics behind the turtlebot calibration?

edit

Because of problems with the calibration, I try to understand what is happening in the turtlebot_calibration node, specifically the scan_to_angle.py, method scan_cb(). This is a piece of code that attempts to calculate a single angle from a scan. The code is:

   for r in msg.ranges:
        if angle > self.min_angle and angle < self.max_angle and r < msg.range_max:
            x = sin(angle) * r
            y = cos(angle) * r
            sum_x += x
            sum_y += y
            sum_xx += x*x
            sum_xy += x*y
            num += 1
        angle += d_angle
    if num > 0:
        angle=atan2((-sum_x*sum_y+num*sum_xy)/(num*sum_xx-sum_x*sum_x), 1)
        res = ScanAngle()
        res.header = msg.header
        res.scan_angle = angle
        self.pub.publish(res)
    else:
        rospy.logerr("Please point me at a wall.")

Can someone tell me more about the mathematics behind this?

You may also notice that the algorithm uses the atan2() in a unintended way, causing a divide by zero in case of all x = 0 (unlikely) or num = 1.