The problem with using the FW values for hardness is that they aren't going to be the same once the salt mixture has been added. All commercially available mixes contains buffers, calcium, will change the alkalinity, ect. Once the SW has been prepared, you can determine what specific minerals/trace elements might be deficient, and possible change to a complete filtration system if needed, but to change before, without knowing if it's needed in combination with the salt mix, seems a waste to me. What if you use the filtered water, and then have to increase the hardness back to the same value? His tests don't show anything abnormal, other than the nitrates, so why alter chemistry when all you really want is cleaner?