20 Min Test - Judging Quality

To @FRANK’s point, if you went as hard as you can for the whole 20 minutes, that little 20% bump in the last few minutes would be very hard to manage. Although it’s always tempting to get up and give it for the last minute of a test (and the first minute, too), IMO it’s important not to do that. You should pace yourself so that you simply cannot give any more at the end. Your cadence should be flagging, you should be shifting gears often searching for the right one, and at the end of the test you should be empty. These are the sensations of a
maximum effort. An example of what empty means: the last several times I’ve tested, I literally had to get off my bike and lay down on the side of the road. A good pacing tip: with 25% time left in the effort, you should be questioning whether you can keep up that pace.

That said, IMO it’s better to err on the side of caution when setting zones. If you set your FTP too high your aerobic work will be too hard and it won’t have the intended effect and you will progress toward overtraining. If your FTP setting is too low, you will quickly realize this while training, and you can give yourself a little bump up until the training “feels right”. It takes a lot of experience to pace a test correctly, so don’t get too focused on the bark of the trees when you’re really need to look at the forest! Good luck!