I calculate my salts by total batch size, typically five gallons, and add that concentration directly to the strike water in the mash. For me, the first priority is getting the right mash pH.
Brewer's Friend advanced is typically what I use to calculate mineral and water adjustments. It's flexible enough to adjust calculations for the mash or total amount of brew water used. You can select a water profile based on style, treat the water for that, and even add your grain data to derive a wort estimate. I'm still learning by plugging "test batches" in that may never be brewed, but it allows me to see how things can affect certain results.