Monthly salt use = pounds of salt per regeneration × number of regenerations per month. The best way to reduce salt use is correct sizing and an efficient salt dose—not simply buying the largest softener.
Simple example
If a softener uses 6 lb of salt per regeneration and regenerates four times per month, monthly salt use is about 24 lb.
High hardness increases salt use indirectly
Harder water consumes capacity faster, causing more frequent regenerations unless resin capacity is increased.
Salt dose changes efficiency
Running maximum salt dose can extract more total grains from the resin, but usually at worse grains-removed-per-pound-of-salt efficiency than a moderate setting.
What to monitor
Track actual bag usage over two to three months, regeneration count and hardness leakage. That gives a better operating picture than the controller setting alone.