One major assumption in this calibration is that the pH probe has deteriorated linearly, which may or may not be the case.
To calibrate, you will need two sets of solutions with an already known pH, preferably ones that aren't too close together.
Take the SensorValue measured at each pH level.
ph1 and ph2 will be the two known pH levels, and SV1 and SV2 will be the measured SensorValues, respectively.
The formula is going to be in the form of y = m*x+b, where y is the pH and x is the SensorValue.
To obtain m:
m = (ph2 - ph1)/(SV2 - SV1)
To obtain b, substitute either the set of ph1 and SV1, or ph2 and SV2:
b = ph2 - m*SV2
And then you can use:
pH = m*SensorValue + b
instead of the formula in the product manual.