A simple question but one I think needs clarification. Can you use bottled mineral water - straight out of the bottle - to top up the boiled wort to a required volume - before pitching the yeast in the fermenter? It will alter the OG of course but the calculator on here will solve that problem. You can then adjust for your efficiency being out from the recipe calculations. I seem to remember reading - somewhere (but I may be mistaken) - that you should only use pre-boiled i.e. guaranteed sterile water but if you can drink the water straight out of the bottle is it not safe to use at any stage - even making a cold starter? The same goes for tap water but then you may not know what the chemical constituency is. Does it matter? Am I going mad due to the covid lockdown? Probably but don't answer that.