EPA secondary guidance
EPA lists 0.3 mg/L for iron and 0.05 mg/L for manganese as secondary (non-health-based federal) guidance values associated with aesthetic effects such as staining, color and taste.
Why we do not “just size a bigger softener”
Some softeners can handle limited dissolved iron or manganese under specific conditions, but the acceptable concentration and pretreatment requirements vary by product. Oxidation/filtration or other treatment may be more appropriate.
Best next step
Confirm the lab result, pH and source. Then use manufacturer limits or a qualified treatment professional for the final equipment train.
Next step
Run your numbers through the Water System Planner, then verify any product against its actual service-flow, pressure-drop, capacity and certification data.