I use power – but same concepts – defer to step two of the plan…. https://www.8020endurance.com/intensity-guidelines-for-8020-running/
Personally, if I get into a test week and I am feeling super blech that week, I’ll just run the 20 minutes in the zone it tells me to stay in (ie – zone 3) – at least that way, I know if I’m maintaining. Then usually every other test period ~ 8 weeks- I’ll do the full FTP test as listed under the intensity guidelines to see if it warrants a “bump” in threshold – not sure if that is a good way to approach it, but either way still ensures that I’m abiding by the intensity guidelines