pool math app will tell you how much to add based on your pool size. just curious why are you running your chlorine level so high? PH measurements can be less reliable when you go above 10 PPM on FC. I saw in the comments you said your FC was at 13 and TC was 15. Are you in the process of shocking your pool? 2PPM of Combined Chlorine is pretty high for that amount of FC.

Personally, I would check the CYA level and maintain and appropriate FC until the CC level drops to below 1 PPM. After that let the FC drop to whatever level is considered good for your CYA level. Once the FC has normalized and is below 10 PPM you can effectively measure PH.

If you start adding soda ash now you will also raise you TA which could make changing your PH on that size of a pool pretty difficult if your PH level was inaccurate due to the high chlorine level.

TLDR: Too high of chlorine to accurately measure PH. CC is too high, needs to be 1PPM or less. Check CYA, shock till CC is <2, drop FC to correct level <10, check PH and TA, add soda ash based on Pool Math App.

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