You will need a small programe called FRAPS which you can get here Birdy.
http://www.fraps.com/download.php
Install it to Programs and then start it before you start the game. You should see a little yellow counter in one corner in your game screen. To start recording, press F12 after 5 seconds into Black Death. At 2 minutes, 30 seconds, press f12 again to stop recording. Exit game and go to start/search/ and enter frapslog. Open the log and you should find your average/min/max