Determine Exact Upload Speed?

I wanted to check the upload speed of the system.

void CheckUploadSpeed()
{
    using (var wc = new WebClient())
    {
        IPv4InterfaceStatistics ipis = networkInterface.GetIPv4Statistics();
        BytesSentb4Upload = ipis.BytesSent;
        FileStream stream = File.OpenRead(string.Format("{0}speedtext.txt", path)); //speedtext.txt is a 5 MB file.
        var fileBytes = new byte[stream.Length];
        stream.Read(fileBytes, 0, fileBytes.Length);
        stream.Close();
        startTime = Environment.TickCount; 
        wc.UploadDataAsync(new Uri("http://www.example.com/"), fileBytes);
        InternetSpeedResult = "Data upload started. Uploading 5MB file";
        wc.UploadProgressChanged += new UploadProgressChangedEventHandler(UploadProgressCallback);
        wc.UploadDataCompleted += wc_UploadDataCompleted;
    }
}

And on Upload Progress Changed

void UploadProgressCallback(object sender, UploadProgressChangedEventArgs e)
{
    InternetSpeedResult = string.Format("Checking Upload Speed ... ");
    double endTime = Environment.TickCount; 
    double secs = Math.Round(Math.Floor(endTime - startTime) / 1000, 0); 
    if (secs >= 30) 
    { 
        UploadComplete(sender, e); 
    }
}

This code actually is serving my issue but the problem is that this is not giving the exact results everytime. Since I am counting on Complete BytesSent in the particular period of time. This numbers automatically varies. If the Speed is very low (less than 512 KBPS) and very high (Greater than 20 MBPS) than its not giving the expected upload rate.

1. What should I be doing in the code that I can rely on the Results?
2. Is there any other approach to check the Upload Speed
3. If the Speed is very low (less than 512 KBPS) and very high (Greater than 20 MBPS). What approach should I take?