Wie berechene ich die Download Geschwindigkeit?

Imports System.Net

Public Class Form1
    WithEvents client As WebClient
    Dim currentBytes As Long = 0
    Dim lastBytes As Long = 0
    WithEvents tm As Timer

    Private Sub btnDownload_Click(sender As Object, e As EventArgs) Handles btnDownload.Click
        client = New WebClient
        tm = New Timer()
        With tm
            .Interval = 500
            .Start()
        End With
        client.DownloadFileAsync(New Uri("http://domain/download.iso"), Environ("Temp") & "\test.iso")  
    End Sub

    Private Sub client_DownloadProgressChanged(sender As Object, e As DownloadProgressChangedEventArgs) Handles client.DownloadProgressChanged
        pBar.Value = e.ProgressPercentage
        currentBytes = e.BytesReceived
    End Sub

    Private Sub btnStopDownload_Click(sender As Object, e As EventArgs) Handles btnStopDownload.Click
        client.CancelAsync()
        tm.Stop()
    End Sub

    Private Sub tm_Tick(sender As Object, e As EventArgs) Handles tm.Tick
        Dim speed As String
        Dim lngDataPerInterval As Long = currentBytes - lastBytes
        If lngDataPerInterval < (1024 * 1024) Then
            speed = Math.Round((lngDataPerInterval / 1024) / (tm.Interval / 1000), 0) & " KB/s"  
        Else
            speed = Math.Round((lngDataPerInterval / (1024 * 1024)) / (tm.Interval / 1000), 2) & " MB/s"  
        End If
        lblStatus.Text = speed
        lastBytes = currentBytes
    End Sub
End Class