A little tough brewing love here: Forcing your mash pH into the ~5.2 range with an acid addition may work for that specific number but the pH itself is not necessarily as important as how it is achieved. If your current pH problem and its' corresponding low efficiency is caused by an imbalance of brewing ions in the water being used for certain beers (which it likely is) the acid is not going to make those problems go away. Having those important brewing ions in the correct quantities for the general style of beer being brewed will, in the vast majority of cases, also provide that the pH of the mash falls into the proper range. It will also mean that your brewing water has enough of the important things like Calcium which has a huge impact on the workings of the mash and not a lot of the bad things that will contribute to screwing it up.