As well as the drivetrain, there's also the issue of the tyre on the drum (it's a cheapo wheel-on trainer). Guessing isn't a problem for me right now as I finished my previous plan before Christmas, and I'll be starting a new one next week with a new FTP test. I guess I could hook the pedals to my headunit and my speed/cadence sensor to Trainer Road for "Virtual Power", and then compare the output at the end, but I'm not sure there's any benefit in working out the delta.