I've learned a few things as I've studied and practiced measuring it. First, in spite of the ability of some meters to do ATC (automatic temperature compensation), it's important to take your readings at something close to "standard temperature" which is 25 degrees celsius, or 77 degrees fahrenheit. Apparently, using the probes at mash temperatures will shorten the life of the probe considerably. Additionally, it seems (for reasons I don't yet understand) that there's still a shift from pH at mash temps to pH at room temp. In other words, yes, your mash temp reading will be correct at that temperature (and the ATC will allow you read at that temperature), but if you were to cool it to room temp (25 C), you'd see a shift in the pH. Brewing texts which discuss optimal pH ranges are based on pH at that 25C temperature, so if you're going to compare pH, you need to have a consistent reference.
I picked up some lactic acid at MyLHBS (much less expensive than I'd expected... something like $2.99 for a bottle) and will be playing with that, as well as continuing to incorporate acidulated malt (which I believe is acidulated using lactic acid, so basically I'm doing the same thing either way). As Hop suggests, water chemistry and pH are intrinsically linked, so before I can really predict mash pH (as opposed to reading/reacting), I'll have to know my water. I'm planning to send a sample to Ward labs, but wonder if I need to time it particularly well in relation to major rain events to avoid getting anomalous readings. Ideally, I'd send a test every 3-4 months and then know what "summer" water is like as compared to "winter" water, but I can't decide if I want to go that crazy yet...
(as far as the beer from the brewday: Although I'm getting better about documenting parts of my brewing, I'm still a little lax with others (like volume measurements in the kettle), so it's tricky to be too precise with some of my efficiency measurements. All I know is that with about 20 minutes left in my boil, I was showing around 19 Brix in the kettle, which is roughly 1.079. I added around a gallon and a half of water, plus some extra wort I'd kept that didn't fit into the kettle (which probably included fermentables, so it may have defeated my purpose a little bit), and managed to get to 1.070 with the wort completely to the top of my keggle. Before I pitched the yeast, I added around 1/2 gallon of boiled and cooled water, and dropped my reading to around 1.062 (slight variation from fermenter to fermenter), but that's way more beer at a higher gravity than I'd planned.
