What you are doing is covering yer butt by dosing at 2 extra lbs per cuft of resin. In this case (the 4 cuft you mentioned) that would add 8 lbs of salt to each regeneration because you're not sizing correctly but...
Based on what you've said about 70K (the max operating K for an "80K" softener is 75K), you aren't getting that 70K with much less than 45 lbs of salt per cuft per regeneration.
BTW, the salt efficiency is 75/45 = 1666.66666 grains per lb. The 80/24 = 3333 grains per lb and that means a substantial salt savings over time plus, fewer regenerations which saves water too if you know how to set the minutes for each cycle position properly, and your control valve allows it.