If you use the entire capacity, which means you must set the salt dose at 15 lbs per cuft of resin you get 2000 grains of softening per lb of salt used, that's 30K in a 1.0 cuft (30,000 divided by 15= 2000). Compared to setting the salt dose to 6 lbs and getting 20K which is 3333 grains of softening per cuft of resin. That's an increase in salt efficiency of 1333 grains per lb. for every regeneration. I.E. if you went to a 1.5 cuft, at 6 lbs/cuft which is 9 lbs, you regenerate 30K where with a 1.0 cuft you need 15 lbs, or 6 more lbs for the same 30K of capacity. Plus, you never want to use all the capacity or you have to use 15 lbs per cuft to get all the resin regenerated again. It's like waiting to buy gas until you run out, it is not good to do that and can cause engine damage.
Can you tell me how you got .75 ft out of what I said about SFR?
Let's say you have 2 showers running (5gpm) and a toilet is flushed (1.5gpm) and someone starts the laundry washer (3.5gpm), your peak demand is the total of whatever is running so 10 gpm in my example. You would overrun the 1.0 cuft 9 gpm and not get all the hardness out of the water. And I'd be telling you that you should have bought a 1.5 cuft (12 gpm SFR) and set the capacity to the same as the 1.0 cuft and the salt to whatever lbs at 3333 grains per lb.. 2*60gals/day/person= 120* 8= 960* 8 days = 8640 rounded to 9K. That would be 1125 gals minus one day of 120 gals for the reserve with a meter setting of 1000 gals. 9000/ 3333= 3 lbs of salt per regeneration and that gets you a regeneration on average every 8 days with a constant SFR of 12 gpm. Then if you have a child, redo the math with 3 people at 60 gals per person etc..