IMO those potentiometers allow only for small adjustment (maybe few, maybe 10 %).
Your earlier post about AC line motors are not for 3.5' drives
- i think that all 3.5' use BLDC with proper motor driver - also what is really important - some of those fdd can generate Index pulse based only on FG signal - they don't need any magnet or Hall sensor because they detect state of the magnetic field on flywheel