The SS495 is a lineair HALL sensor (magnetic sensor).
At a 5v voltage its zero point has a 2.5v output. Depending the polarity of the magnetic field the voltage will go up or down and how much is regarding the strength of it. The maximum of 640 gauss has a 2v range.
The used meter works at 9 up to 12 volt so a 5v regulator is needed to make it work on a 9v battery. The meter has a 200mV range and with 2 resistors I could use the full scale of the meter.
The potmeter is just a zero adjust before placing a rotor.
(Be aware the used panelmeter must not have a shared ground from supply voltage to input)
I mounted the sensor at about 5mm distance of the rotor so the magnetic field did show half scale.
Any other rotor will show different values so you can dterm the strength of it and select the best one out of a few.
This thing is made to detect illegal rotors with more magnetic field into a 1 brand/model stockmotor. The motors measured were in between a range of 990 up to 1017. The 1017 was from a person with the fastest motor on the track so yes, a small improvement on the magnetic field can give more performance.
So it does not show a strenth on a right scale but it will show differences between rotors.