10 gal is about 38 L and you need 1.72 mEq/L for a total of 65 mEq. With 10% phosphoric delivering 1.1 mEq/mL that comes to 65/1.1 = 59.4 mL. You will acutally need less, about 90% (~ 54 mL), as you don't have to overcome the water's entire alkalinity (wrt pH 4.5 or so) but only the alkalinity wrt 5.4.
Be aware as you add this that the pH will change slowly at first and then much faster as the pH approaches. So initially add 30 mL and then add the rest in small increments, stirring and checking as you go. If you overshoot just add more tap water. It isn't necessary to be spot on.
Yes, you will need the additional acid or sauermalz for the grain's alkalinity. Since you have the meter it is a great idea to make a mini-mash with the treated water and about a pound of the well mixed grist to see where you will come out in pH. Any change in acid is then scaled and applied to the full mash.
