A softener that is forced to remove iron and Manganese, cannot operate as efficiently as a softener where I & M has been removed from the water prior to entering the softener.
Manganese will often create greater resin fouling problems than iron.
Recommend basing calculations and settings using 5 gpg capacity consumption for each 1 PPM iron and Manganese.
Mn (1.5 ppm) + FE (0.5 ppm) = 2.0 ppm X 5 = 10 compensated gpg + 10 gpg actual hardness = 20 gpg X 1.1 hardness compensation = 22 gpg total compensated hardness.
The higher hardness setting will cause the softener to regenerate more frequently than a lower hardness setting. More frequent regeneration will be necessessary when a substantial quantity of FE &/or Mn is removed by the softener.
The usable capacity setting will be dictated by the salt setting that is programmed.
For example, since your 12" X 52" media tank will contain 2 ft3 of resin, a 6 lbs per ft3 salt setting will mean 12 lbs salt total, which will regenerate 42,000 grains usable capacity each Regen cycle.
8 lbs/ft3 will mean 16 lbs salt will regenerate 48,000 grains usable capacity each Regen cycle.
See chart below regarding how the salt setting will dictate the usable Capacity, Hardness Reduction Efficiency and water quality.