The short answer
Enter your hardness in grains per gallon plus about 4 for each ppm of ferrous iron; setting it far higher only wastes salt.
The number to enter
Compensated hardness = hardness in gpg + 4 × ferrous iron ppm (Nelsen). 15 gpg with 1 ppm of iron is entered as 19. Convert mg/L with the hardness converter.
What the setting does
On a metered valve, a higher hardness setting means fewer gallons between regenerations, more salt and more water. Too low, and hard water comes through before the next regeneration.
Check it worked
Test the water at a tap with On The Go 481108 Hardness Test Strips a few days after setting it; it should read near zero.
Products mentioned
Questions readers ask
What hardness setting should I use on my water softener?
Your measured hardness in gpg plus about 4 for each ppm of iron.
Why does my softener regenerate so often?
The hardness setting may be above your water's, or the unit is small for the household.