Net water, not tank rating. Use the Tank Volume calculator if you have not measured it.
Potassium phosphate to bring phosphate up off zero, in the very small quantities this actually needs.
Raises 284 L by 0.060 ppm phosphate. The dry mass is milligrams, which no kitchen scale can weigh. Make the stock solution and dose by volume.
grams = (0.060 − 0.000 ppm) × 283.9 L ÷ 697.9 = 24.4 mg
Monopotassium phosphate KH₂PO₄ has a molar mass of 136.084 g/mol carrying one phosphate ion at 94.970, a mass fraction of 0.6979. One gram in one litre gives 697.9 ppm phosphate.
The numbers here are tiny because reef phosphate targets are tiny. Taking 400 litres from 0.00 to 0.06 ppm needs 34 milligrams. That is why every practical version of this uses a stock solution.
Rock absorbs phosphate and gives it back slowly. A tank that reads zero often has a lot bound up in the rock, and dosing chases a moving target for weeks. Dose small, test often, and expect the number to drift back down at first.
Phosphate at true zero is associated with tissue loss in SPS, with cyanobacteria, and with dinoflagellates. Most reefers aim for 0.03 to 0.10 ppm rather than the lowest reading they can get.
Commercial versions of this exist and cost far more than the salt does. They are the same compound in water, which is worth knowing before buying a bottle.
Sources: Dieck, Reef Chemistry Calculator