Really no way to answer this question based on the information provided. Tap water pH is somewhat meaningless, to expand on what @uncdeo posted. More important is the mash pH, which is dependent on your water alkalinity, ion concentration, and actual grist. The water's pH does not provide any meaningful information on alkalinity.
In general, I find it easier to use lactic or phosphoric acid to adjust pH, since acid malt can be variable and it's tough to make adjustments with it. Much easier to add a little bit of lactic acid until you get the mash pH you want.
It is also important to control sparge water pH, in that you can extract tannins if your sparge pH gets too high, like 6.0 and higher. For adjusting sparge pH, acid malt isn't a great option.
Best option is to get a Ward Labs water report. Household chemicals is good enough, and it'll save you a few bucks over the brewing water report. Then plug your water profile into a brewing water calculator such as Bru'n Water, along with your grist. This will get you very very close to the true mash pH, and will let you dial in the amount of acid malt or lactic acid to add to the mash. It also will tell you how much acid to add to your sparge water.