I've been using BeerSmith for years but have never created anything using a phone or tablet. Always use the pc.
So I'm not sure how that works, but on the pc, every recipe has a "Water" tab where all the magic happens.
You create a profile for your base water (in my case, RO) with the pertinent chemistry numbers (from your water test), add the amount of water necessary for the batch (which will also show up at the top of your Design page), decide what actual brewing water profile you want to use (click the "Match a target Profile" button towards the right side of the page and pick one from the pop-up list box), and at that point BrewSmith will generate the additions needed to hit that chosen profile.
I do not actually use this BeerSmith function but do compare it against my Bru'n Water spreadsheet just out of curiosity. With the notable exception of acid additions they do tend to be in sync...
Cheers!