What you're trying to control with salts in the mash is the mash pH. The water calculator here takes your grist, your source water, your salt and acid additions and comes up with a predicted mash pH value. This should be at or near a value of 5.4, a bit lower for very light beers, a bit higher for very dark ones. That's one function of salts. In other beers, salts are a part of the flavor profile. The calculator doesn't work for those - you'll have to discover that by adding salts to the kettle. Don't worry about the salt values or the water's pH, they're mostly irrelevant. Concern yourself with the mash pH.
Salts reduce residual alkalinity - the quantity you can use to predict your mash pH - as well, just not as much. General rule: Adjust your salts to where you want them then finish adjusting pH with acid.
