I think it depends on what you are going to use the number for. If it's just a number, and you want to see if you are getting stronger, then pick one and stick with it. I use trainerroad, and all the workouts are scaled to ftp. I found that if I stuck with what I tested, the workouts were too hard and I didn't get anywhere. So I set my ftp to a lower number and I think I am making more progress now. I think part of my problem was that I chose the 8 minute test, and I'm more suited to that test.