Net water, not tank rating. Use the Tank Volume calculator if you have not measured it.
Calcium chloride or a commercial calcium supplement, sized to a target in ppm.
Raises 284 L by 40 ppm calcium using Calcium chloride, anhydrous CaCl₂. Never dose calcium and alkalinity into the same spot at the same time. Separate them by an hour or add them at opposite ends of the system.
dose = (440 − 400 ppm) × 283.9 L ÷ 361.1 = 31.4 g
Calcium chloride dissolves into free calcium ions, so the math is a mass fraction and nothing more.
Anhydrous CaCl₂ has a molar mass of 110.978 g/mol and calcium is 40.078 of that, a mass fraction of 0.3611. One gram in one litre therefore adds 361.1 ppm calcium.
The dihydrate CaCl₂·2H₂O weighs 147.008 g/mol for the same one calcium, a fraction of 0.2726, so one gram in one litre adds 272.6 ppm. You need a third more of it by weight. Check which one you bought, because prilled and flake products are usually hydrated and the bag rarely says so plainly.
Calcium chloride flake sold as ice melt is typically 77 to 80 percent CaCl₂, roughly the dihydrate. It works, but the food-grade and reef-grade versions are worth the small premium for a tank with fish in it.
Dosing calcium alone does not hold. Calcium and alkalinity fall together as corals build skeleton, and a tank that needs calcium almost always needs alkalinity in the ratio of 7.16 ppm calcium per dKH. Correct both.
Adding a large calcium dose to a tank with high alkalinity can precipitate calcium carbonate, which turns the water cloudy and drops both parameters at once. Keep single corrections modest and separate calcium from alkalinity in time and place.
Sources: Seachem Reef Advantage Calcium · ESV B-Ionic label · Brightwell Calcion · Red Sea Foundation manual (PDF) · Dieck, Reef Chemistry Calculator