6 lbs salt to regenerate 18,000 grains capacity in 0.75 ft3 resin, is equivalent to 8 lbs to regenerate 24,000 grains capacity in 1 ft3.
8 lbs per cubic foot is the usual recommendation for a residential softener as it offers the best balance of Hardness Reduction Efficiency, Water Quality and usable capacity. As efficiency is increased, water quality (hardness leakage) and usable capacity will be decreased. Some that desire higher efficiency, are often advised to use no less than 6 lbs/ft3 due to the resulting lower water quality after repeated regeneration cycles.
Here is a chart which Ditttohead had prepared.
https://terrylove.com/forums/index.php?attachments/resin-chart-jpg.54819/
The HR Efficiency, WQ and hardness leakage amounts for each salt setting, are indicated at the bottom of each salt quantity column.
While 75 gallons/day/pp had been a common estimate amount, with modern efficient appliances, low flush toilets and other water reduction methods that are now common, 60 gals/pp will often provide a more realistic estimate. With metered softeners that are not removing iron, there is usually no concern with overestimating water use and installing a slightly larger softener.
When the water does not contain iron, there is usually no concern with up to 30 days for a regeneration frequency.