Net water, not tank rating. Use the Tank Volume calculator if you have not measured it.
Measure how fast the tank burns alkalinity, then size the daily dose that holds it flat.
This tank burns 0.50 dKH a day. Dosing that much daily holds alkalinity flat where it is. To move it, use the alkalinity calculator for the correction and add it on top of this maintenance dose.
daily dose = 0.50 dKH × 24 ÷ 24 h × 284 L ÷ 33.33 = 4.26 g/day
Consumption is measured, not looked up. Test alkalinity, dose nothing for a day, test again at the same time. The difference divided by the elapsed hours is the demand rate.
Test over 48 or 72 hours rather than 24 where you can. A Salifert alkalinity kit resolves about 0.1 dKH at best, so a 0.3 dKH daily drop measured over one day is barely above the noise. The same drop over three days is 0.9 dKH and unmistakable.
Once demand is known the dose is the same arithmetic as any correction: demand in dKH times volume in litres divided by the product strength.
Splitting the daily dose into many small ones matters more than the total. One large dose swings alkalinity up then lets it fall all day. Twenty four hourly doses hold it nearly flat, and corals respond to stability far more than to the average.
Calcium demand follows alkalinity at 7.16 ppm per dKH, from CaCO₃ stoichiometry. Dose both. A tank dosed only for alkalinity runs its calcium down until something precipitates.
Demand rises as corals grow, so re-measure every couple of months. A dose that was right in spring will be short by autumn on a growing SPS tank.
If alkalinity keeps falling despite dosing the measured demand, check magnesium first. Below about 1250 ppm, calcium carbonate precipitates on its own and no dose keeps up.
Sources: Dieck, Reef Chemistry Calculator · Holmes-Farley, DIY two part (Reefkeeping 2006)