You should not go seven days between regeneration and you should not use a system to the maximum capacity. The units have a maximum capacity stated just for sales. When you take a cu. ft. of resin and regenerate it with 15 lbs. of salt, it will give you 30K grains of softening capacity. (32K is under a laboratory condition using water with only hardness in it. You will never encounter this in real life.) You can take that same cu. ft. of resin and regenerate it with only 6 lbs. of salt and get 20K grains of softening capacity. Therefore why would you NOT regenerate it twice and get 40K softening capacity for only 12 lbs. salt. This will not waste salt or send it to the city to treat as waste water. Therefore the example you gave, you could go with a 24K, just regenerate it more often at a lower salt setting.