A reading is off and you want the specific correction, not a lecture. Pick the problem; each page gives the steps and the exact amount for your tank.
Open the full calculatorThe most expensive corrections in this hobby are the ones made against a bad test result. Before acting on a reading that is far from where the tank has been sitting, run it again, ideally with fresh reagent. A genuine swing and a fumbled titration look identical on paper and only one of them needs a dose.
Every fix here splits across days when it exceeds the safe daily limit. Resist the urge to chase the number back in a single evening, and change one parameter at a time so that when the tank responds you know what it responded to.