Net water, not tank rating. Use the Tank Volume calculator if you have not measured it.
Alkalinity and calcium should track each other. When they do not, this says which one is actually wrong.
Calcium sits 51 ppm above where this alkalinity puts it. Alkalinity is the parameter to raise. Dose alkalinity only until they line up, then resume dosing both.
expected Ca = 440 + (7.00 − 8.50) × 7.157 = 429 ppm · measured 480 ppm · off by 51
Alkalinity and calcium are tied together by the mineral corals build. Every dKH of alkalinity consumed takes 7.157 ppm of calcium with it, from the CaCO₃ stoichiometry: one mole of calcium per mole of carbonate, which is two equivalents of alkalinity.
So the two parameters move along a line, not independently. Given a target pair, the calcium you should see at any measured alkalinity is Ca_target + (alk_measured − alk_target) × 7.157. The gap between that and what you actually measure is the imbalance.
The practical use: when both read low, dose both. When one reads high and the other low, dosing both makes the gap worse. Raise only the one that is behind until they line up.
A persistent imbalance usually has a cause worth finding. Calcium climbing while alkalinity falls happens when only one part of a two part system is being dosed, or when one dosing pump has drifted. Alkalinity climbing while calcium falls points the other way.
Magnesium overrides all of this. Below about 1250 ppm, calcium carbonate precipitates spontaneously and drags both parameters down no matter what you dose. If magnesium is low, that is the whole answer and the imbalance is a symptom.
A twenty ppm gap is within test kit error and not worth chasing. Chasing noise is how tanks end up swinging.
One more suspect: a persistent imbalance sometimes means the test kit rather than the tank. Cross-check with a second kit or an ICP test before making a large correction.
Sources: Holmes-Farley, DIY two part (Reefkeeping 2006) · Dieck, Reef Chemistry Calculator