We will need to have a power curve for each separate resistance (knob)
So if you have one that you can use most of the time, that would be good.
Then you could record the trainer speed and the power at the same time (with a power meter and a speed sensor)
After a few workout, when we have a lot of data, we can check the .fit file and build a spreadsheet that will have
5 kmh = 150 W
10 kmh= 200 W
The more points the better to derive the formula!