Burton salts (as used for homebrewing, to match the mineral content of the water of Burton-on-Trent: <a href="https://www.morebeer.com/products/burton-salts-brewing-1-lb.html" rel="nofollow">https://www.morebeer.com/products/burton-salts-brewing-1-lb....</a> ) are a pretty close match to San Pellegrino at 1.2g/L. You want the salts to be dissolved in the water before carbonating it. And of course, this might not work well if you have highly mineralized tap water to start with.
Imagine my surprise when I clicked the "a mineral water calculator" link and my browser downloaded an Excel spreadsheet. It'd be cool to port this to the web.