Wow, you guys seem to be going to a lot of trouble to fix a simple problem. The amount of SMB or KMB needed to dechlorinate 20L of tap water is contained in about a quarter of a Campden tablet. The amount of of Na or K is negligible. It's within the uncertainty of the water analysis. The taste...
The amount a acidulated malt (or a lactic acid addition) should be dictated by your water profile. And maybe a bit extra if you are looking to simulate the Guiness "tang".