Interesting thing… I occasionally use intervals.icu and they have a metric based upon past performance data: eFTP**: So when I did my 20 min ftp test, which was a time trial on zwift, the net result was within 1 watt of each other.
“**Estimated FTP is calculated using power curves from FastFitness.Tips and Morton’s 3 parameter critical power model. The algorithm requires just 1 maximal effort of between 180 seconds and 30 minutes. Your estimated FTP will stay the same if you maintain your training load and decline slowly when you reduce training. To get it to go up you need to put in a max effort! The minimum duration required is configured in settings.”