Check out this great free spreadsheet for water tweeking. It allows you to make additions to the mash and/or the sparge. You can use RO water for just the sparge, all kinds of other cool things. At the bottom is shows RA and Cl:SO4 ratio and has a guideline next to each which shows what that value corresponds to. In the case of RA the SRM (color) of your beer, in the case of Cl:SO4 it shows if the ratio is good for bitter or malty brews.