The op asked specifically about 20 minutes, however, the same could be said of any test. I think pacing for 5 to 10 minutes is insanely more difficult than 20-40 etc…IMO, taking an estimate (algorithm of PDCs?) of an estimate (.95 of 20 min) used as a proxy for maximum lactate steady state, seems to me, like the whole point of testing is getting lost.
I guess I’m in the minority or a bit old school. I want a number to work zones to push the PDC up and right to compete. Getting familiar with riding longer maximum efforts are essential to this. Also, since my ftp seems to fluctuate 40w in a season doing the same test the same way seems critical.