You have a water hardness of 30 and want to reduce the water hardness to 12. Your spa has a water volume of 1200 litres. You then take 2.1 kg and multiply it by 1.2 (1200 litres). Result: 2.52 kg. You divide the 2.52 kg by 0.8 kg (weight of the Calc Down Pad) The result is 3.15. In this case you need 3 Calc Down Pads as one pad contains 0.8 kg softener mineral.
You can also take 2 Calc Down Pads and soak them (when the water hardness no longer decreases) for a few days in distilled water with 30% salt content. This removes the calcium ions from the softener mineral and replaces them with sodium ions. The Pads can then be reused.