People talk about malt's buffering capacity, usually meaning that it will lower pH from the normal water pH range (7-8) down to mash pH. I guess I should've known the buffering would work both ways.
I overshot my acid addition on my latest lime-softening batch of water, and my water's pH got down to 4.0. I plugged all the numbers into Bru'n water, and it estimated a mash pH of 5.2 without any lime. I added one gram, which it estimated would result in a mash pH of 5.3. My actual measured mash pH was 5.4.