pH control is not only a mathematical problem. There are also some chemical/physical issues that you should know about.
1/ to make a pH correction, you have to add an acid or alcali solution. the amplitude of the control action should also be a function of the solution concentrations that are used.
2/ the amplitude of the pH variation is of course a function of the amount of acid or alcali added, but also of the buffering properties of the medium. Adding 1 cc of a 1N NaOH solution to 1 liter of water as not the same effect as adding it to 1 liter of 1N sodium acetate solution.
3/ the mixing time should also be taken into account. When acting on a large tank, a fairly large delay between the corrective action and a variation detected by the sensor can affect significantly the efficiency of the control loop
4/ a pH electrode can also have a significant response time, depending on its history.
Of course this is not an answer to your question, but I believe that it will give you a better appreciation of your situation.
CC
Chilly Charly (aka CC)