Here's the deal. You can test your 20 minute power however you want.

As long as you do the test in similar conditions, similarly rested, and in a similar protocol, your numbers will be fine. We're concerned about the trend here.

If you can't do workouts in zones as prescribed by your FTP, then you might be better off not testing that way.

I'd do them the way that kensuf just said after a day off. If you have a trainer, you might want to MAP test as well so you have two different setups going.

Good luck!