Hi Chris,  I was also ...

Hi Chris,  I was also operating a SWRO plant in Namibia some years back, and we had a filter bed of limestone chips that we passed the permeate through before putting it into the distribution or storage containers.  This is not an accurate way to achieve PHs, but it certainly helped.  To determine the LSI for your water, a helpful tool has been developed by the South African Water Research Commission appropriately called STASOFT.  You can download it from their website.  I tried to upload it for you but because it is an exe. file, this website will not allow it to be uploaded.  Please try it - it is safe to download and very useful.