Although a pH meter would be best to empirically determine the amount of acid to add, the size of protein break can be used to determine when your in the ballpark. Flocs will become larger when pH is optimal but may also get smaller when outside your target, high or low. CaCl2 or gypsum can be used as well as acid but keep in mind these may also change the character of your beer in other ways. You can make your additions towards the end of the boil if you want to preserve the utilization of hop bitterness extraction.
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