"170% V02Max during the 20 seconds on"
This is where most people fail. 20s "all-out" is easy when you're so fatigued that "all-out" becomes a crawl. If you're not truely going "all-out" then you're not really doing Tabata.
Try it on a stationary bike in which you can measure Watts (most bikes can these days). Crank it up to a decent resistance and go all out for the first rep and then try to match the same wattage on the successive reps. I say if you drop off by more than 10% then stop the protocol.
So, you first crank out 300 watts for the first rep...
2nd rep: 290 watts
3rd rep: 290 watts
4rd rep 280 watts
5th rep 265 watts...you stop, your workout is done. Your next tabata workout you try to improve, ie: do more reps until you can do all 8 without dropping 10% in wattage.