Formulae 4 Water Softner - Regarding Using Of Resin (In Ltrs), Cylinder Capacity (In Ltrs), Salt (In Kgs).?

My Water Hardness is 500 ppm, Daily water consumption is 400 Liters, Salt Regeneration must be once in 7 days. Please explain what Capacity of Cylinder (in Liters) should I use? How many Liters of Resin should I use? How many Kilograms of Salt should I use for Regeneration once in 7 days. Any one please explain me clearly the formulae how to calculate.