It is based on a SS495A
Working at 5v the zero point is at 2.5v. On one side there is a potmeter to adjust t0 zero. The calculated resistors for the 200mV digital meter is only to use the full range of the meter with the range of the hall sensor. Ideal to measure the differences between several rotors or with stockracing to detect illigal rotors.
If you want the real Gauss scale you need to calculate the right resistors.
The sensor needs to be on a 5~10mm distance of the rotor.