Water balance base line

I'm aware of the tolerance either side of 0.5 regarding the water balance (langelier index) I'm I right to take a base line water balance from the mains water and then aim to keep the pool water within the 0.5 +/- tolerance of the mains? I'm getting a water balance reading of - 1.08 Also is there documentation that tells me the corrective actions (or is it a case of refreshing the water?