Don't do it, 5.2 just adds a lot of sodium to your brew and doesn't adequately promote a good mash pH. From the sound of the OP, they have their water report and have some knowledge of how the mash pH will go. More than likely, the alkalinity of that water is a little higher than desirable for this particular brew. If only alkalinity is the problem, then an acid addition is the preferred treatment.
Unfortunately, that passage in Palmer's book regarding phosphoric acid causing excessive calcium precipitation persists. The mash contains hundreds of times more phosphate ion than will be added by a phosphoric acid addition and therefore adding phosphoric acid does NOTHING to increase or decrease the precipitation of calcium in the mash. Using phosphoric acid in the mash and sparge water is OK.
Using other acids are OK too. Phosphoric may be preferred if the tap water has high alkalinity since it has the least flavor impact of any acid in beer. You might taste other acids like lactic or citric with the amount of acid you might have to add in the case of this high alkalinity water. If its only moderately alkaline, then those acids might be fine too.
You might appreciate the additional guidance and knowledge that Bru'n Water software can provide to help you with acid additions in brewing water treatment.
Enjoy!