I am getting ready for my first all-grain batch. I got a copy of my township's water report and sat down to use it in conjunction with the graphs in How to Brew to determine what colors of beer I can make without pH adjustments and if I want to make other beers, what adjustments I need to make. The problem is the water report doesn't list the Magnesium content. This data is crucial to using the charts. I've emailed my township to try to get the info, but I'm not sure I'll hear back.
Assuming I don't know the Mg content of my water and thus can't predict my mash pH, can I just measure the pH of the mash using a pH meter and adjust accordingly? Is taking a measurement actually better than estimating based on the water report? When should I take the measurement - after doughing in? After X minutes?
According to How to Brew, brewing water is adjusted with salts before you begin mashing. If I can't predict my mash pH and instead have to measure it after starting the mash, how should I add the adjusting salts? Should I mix them into a pint or so of water then stir them into the mash as well as I can? If so, should I consider the start of the mash timer to be when I've adjusted the pH?2.