Hi mate, welcome to the TR community.
I believe zwift uses a different testing protocol which is more susceptible to pacing errors. Not many people know what their 20min max or 2 x 8min max effort is, so it’s generally an exercise in figuring out what you think you can do and then trying to do it. As you can see, unless you have a good basis for your assumption, it’s not the most fantastic way to achieve what you’re after, which is accurate training levels.
What’s important to note however is that although the ramp test takes away a lot of these pacing issues, it is still predicated on an average deviation from your 1 min max power, which is to say that TR takes 75% of your 1 min max and says “that’s your threshold”. For many people, this percentage will be higher, for others it will be lower. The best way of finding out where you sit is to try and do a 2 x 10min threshold workout. You should be able to sustain it without feeling like you want to die or that you’re digging really deep. During the workout, adjust the intensity up or down, and I would use that as your new FTP.