I know I'm resurrecting a dead thread, but just wanted to give my inputs. Relatively new homebrewer, about 6 months of all grain. Used 5.2 for the first time on a batch of Janet's brown. Just had the first taste and it was phenomenal, but had a salty finish. Pretty bummed to be honest, because I've been on a brewing spree and have 3 more beers coming through fermentation that I've done in the past 3 weeks that might have the same issue because I used it on all 3. For reference, I'm using Boston water, which is very highly alkaline and has a very high pH and quite low "hardness". This month we had a pH of 9.6 and alkalinity of 39.8.
An alkalinity of ~40 is not considered high. It's actually fairly low and instead of adding 5.2 I's just add 3% of acidulated malt for any light beer. For your Brown you should have been fine w/o any adjustment.
Water chemistry can be daunting. You can use this nice on-line calculator that I developed for Brewer's Friend: http://www.brewersfriend.com/mash-chemistry-and-brewing-water-calculator/
I think Boston water is coming from the Quabin reservoir. This means it is surface water and these water's tend to be low in Alkalinity and hardness. Adding some calcium salts (Gypsum or Calcium Chloride) may help your beers.