So actual_speed = (specified_speed - 30) * 10 / 7. Specifically, the 0-100% range in PX1 is 'compressed' into the 30-100% range in FanControl. One thing I've noticed that's weird on the 3080 Ti XC3 is that the fan speed percentage in FanControl doesn't match the speed the Precision X1 shows.
I use FanControl for all my desktop's fans (case, CPU, GPU), so it's really nice it works for the GPU's third fan too.