For the most part, the affect of the mineral salts for water quality and mash pH should be based upon the mash and sparge water volumes. This can change depending upon how much you need to top off in your fermenter. In general, it is the minerals, Calcium and Magnesium, which aid in the enzyme activity during conversion and later will help the yeast activity and ability to flock, falling out of suspension. Usually, there is enough Calcium and Magnesium coming from the grains to supply what you need for fermentation.
Having said that, you can adjust for a final wort concentration of added minerals and ions using the water profile tool in BeerSmith. You can start with a typical mash volume, add the dilution water volume as your top off water, and then construct the ionic balance for where you want. Then remove the dilution water volume to get a target for your mash water. Save this as a water profile and then use the water tab in the recipe to match your mash volume to this new profile. Then, once you add in your top off water, your finished profile should be close to where you want it to be going into the fermenter.