There is little relevance to ones source water pH, since the waters buffering capacity (its ability to resist changes in pH) is nill with respect to the vastly much greater buffering capacity of the recipes grist (its various malts and unmalted grains).
If you are using tap water, the first step is to get a lab analysis of it if you are on a well, or to contact your local water company and ask for their analysis if you are on municipal water. You want to know your waters mg/L (ppm) of specifically these ions: Ca, Mg, Na, Cl, and SO4. Also you will critically need to know your waters mg/L (ppm) of alkalinity (reported as CaCO3), or alternately its mg/L (ppm) bicarbonate.
Find a spreadsheet that computes mash pH and assists in its adjustment (such as mine, and yes, I am biased, but at least it is both complete and free), or anyone else's program, and on the water page enter your source waters analyticals and alkalinity (some request bicarbonate here). Also tell the program how much of your water (as a percentage) will be coming from your analyzed source, and how much will be blended in from another source (fully optional, only if desired, with typical alternative sources being RO or distilled). Then on the main page enter your recipes individual malts, and their respective colors, and their malt "class", and lastly add in your desired minerals. Also (for my software at least) enter a target mash pH (with 5.4 being the most common target here). The program will then guide you as to how much acid (phosphoric or lactic) or base (baking soda) to add to "nominally" (+/-, within reason) hit the mash pH target.
But beware that any and all of such mash pH guidance software programs are only so reliable, and are not at all to be considered valid enough to be an outright replacement for actual pH measurement during the mash.
