We would suggest that you turn the ERG mode of your indoor trainer off for all short, Vo2max and sprint intervals. Then you can really push yourself to the maximum without being limited by the ERG mode. The power levels suggested by JOIN are merely a guideline, but if you really want to put the pedal to the metal, turning off the ERG mode is the best option.
For longer intervals, we would suggest leaving the ERG mode on, as they require a more steady effort, usually around the FTP- or Tempo-levels.