I'm not sure our calcium is very low at all, in fact I suspect it's the opposite. I've always wanted to send a sample off to Ward or somewhere else but haven't got around to it. I like the taste of our water, I think it's good. I just make mash additions strictly to control pH, that way I know I'm maximizing my chances of getting good conversion.
I know I kinda said this before but, basically with our water I've found that all mashes, except ones that are heavy in darker roasted grains such as stouts and porters, end up with a pH that's a little too high. 1/4 -1/2 tsp of lactic acid almost always does the trick, I really don't even measure it about half the time because I've done it so many times, I just know. The darker beers will actually have a mash pH that's TOO low. Calcium carbonate will fix that, but it's a mother to dissolve if you just throw it in the mash. My rule is if I think I'm going to need it, I add it as soon as I start heating up my strike water, and stir it pretty often just to be on the safe side. For me 1/2 tsp usually does the trick, but depending on the recipe may take a little more. Baking soda actually works better, and faster at raising the pH, but use it with an even lighter hand. 1/8 tsp to start out with I'd say. Think of it as "pH blaster".
I'm curious as to how the BIAB full volume mash is going to effect pH. The grain itself is what contains the things that help drive the pH where we want it, with more water, those minerals/salts/acids will be diluted more, thus having a smaller effect on the mash pH. How much smaller I don't know. I will let you know what I find out though. I think the answer is going to be just adding a little more lactic acid, but never adding enough where it could actually be tasted or anything. As long as the pH is in the ball park anyway. We're pretty lucky with our tap water. I've read on here about others who, without adding brewing salts or acid, have crazy mash pHs. With ours even if you don't do anything, it's usually damn close to where it needs to be anyway. Your beers tasted like they finished plenty dry enough, and you didn't even do anything to mash pH I'm guessing. More proof that barley wants to become beer!