Originally Posted by
southernfox
But this is true on the road, too, depending on what road one is doing their 20' test. So I don' t think it's an issue at all.
This is very true. There is never a perfect test. Once you do several over a period of time you'll get a sense of your FTP and the pacing you need to get an accurate result. Don't overthink it, just do some tests. If they vary by several watts, that's fine. It's just a number to give you guidance for your various training zones.