Well yes I’m more inclined to believe what my Stages shows me than what Tacx is displaying.
The Stages is measuring strain at the crank but Tacx are calculating based on the resistance and how fast they see the flywheel spinning, but there are many assumptions there that could affect that calculation that it’s pretty much a guess.
Afraid I can’t help map the levels to the Fortius unfortunately. My unit has controllable resistance from -4 to +9.
I believe that with virtual power they have a curve for my trainer with resistance +4, but I have had a power meter for a few years now so don’t remember exactly.