Members of the American Water Works Association (Water COs across the US) say their customers use 60 gal/person/day. They are the ones with water meters on many millions of houses. Plus shower heads, toilets, faucets etc. are all water conservation types in most homes today. I used their figure for some 20+ yrs and checking with customers that had a control valve that measured and recorded daily water use showed 60/gals/day/person was right on. Now they didn't have anyone taking 2 showers a day or wearing clothes for a few hrs and throwing them in the laundry so it was necessary to run a couple loads of laundry a day...
All softeners are sized by the cuft of resin, and that dictates the size of the resin tank and that dictates the control valve that can or can not be used to service the cuft volume of resin/size of the tank.
The K of capacity is controlled by the lbs of salt used to regenerate the softener/volume of resin and the lbs are adjustable on all softeners although some companies won't tell the customer how to do it.
In the real world, as opposed to on paper... you get a maximum of 30K of capacity per cuft of resin IF you use 15 lbs per cuft of resin. Less salt, less capacity but not the 32, 48, 64 K as many dealers and sales people claim. They are the same ones that don't help their customer size a softener or tell them anything about salt efficiency... 30,000/15lbs = 2000 grains/lb. A 1 cuft set at 6lbs gets 20K. A 1.5 at 9 lbs gets 30K and that is at 3333 grains/lb. 1333 more grains than 2000.
Yes for SFR you add together the gpm flow of those fixtures your family normally run at the same time. A 2.0 cuft has a 13 gpm constant SFR. A 2.5' has 15 gpm. And that is real world not some inflated BS marketing type figures. And that is the flow rate of the entire softener, not just the control valve.