Since the programming and simultaneous serial monitoring with the ATTiny85 was a massive pain, I made a Arduino Nano Every version of the PCB and the program.

Now I can adjust the limits and offset easily while monitoring the values in real time. 

Here's with the plotting of the Vpp value as well as the matching working area of the extractor

The only issue I noticed is for the lower limit : when I unplug the USB cable, the reading from the Hall sensor varies a bit so if the limit is too low the air extractor keeps running on low speed when the AC is off. I increased a little bit the lower limit blindly in order to correct that and that was fine.