Just as your vehicle does not need to run out of fuel before you refill the tank, a softener's capacity should not be exhausted before regeneration occurs. Unlike a vehicle, a softener's 'gas mileage' (salt efficiency) will be higher if a specific amount of capacity remains unutilized when regeneration occurs.
A 48K capacity softener usually contains 1.5 cuft of resin. To regenerate all 48K would require 27 lbs salt every regen cycle, resulting in a maximum salt efficiency of only 1,777 grains per lb.
The same 48K softener will deliver 36K usable grains when regenerated with 12 lbs salt (3,000 grains/lb) or 30K grains with 9 lbs salt (3,333 gr/lb). Either of these configurations is recommended. The 36K/12lb setting will provide higher quality soft water (less hardness leakage) compared to the slightly higher efficiency 30K/9 lb configuration.
For comparison, a 1 cuft (32K total capacity) softener would require 15 lbs salt to deliver 30K grains.
A municipal water system will often obtain water from multiple sources, each with a different hardness amount. The hardness within the distribution system can vary depending on your home's proximity to each water source, time-of-day usage throughout the city and also season, maintenance, etc. As such, it is advisable to anticipate some variance by programming your softener for 2-3 grains higher than the actual hardness test result obtained at your location with an accurate titration test kit such as a Hach 5B.