As Brewtopalonian suggests, we want to keep the pH of the runoff in the range where we won't extract tannins and silicates, generally below 6.0 or better below 5.8 -- in the mash range. In a fly sparge, as the runoff becomes more dilute, the residual buffering power of the mash has less effect, and the runoff will tend to rise toward the pH of the sparge liquor, so it needs to be within the acceptable range itself -- nearer 5.5. In a batch sparge, you never really dilute the wort to where you lose the help of the mash buffering, so the sparge needn't be acidified as much; 6.0 ought to be sufficient. If you're running off in two batches, all you need to do is check that the second, diluted, batch, after stirring in the sparge, is not above 5.8, and you should be able to establish a rule of thumb to go on.
But pH of the liquor is not the be all and end all. It's kind of a red herring. What we really are after is having little or no ALKALINITY in the sparge liquor. With no alkalinity, that is no alkaline buffering in the liquor, the pH of the dilute mash can't be pulled up too high. If you use RO, just don't add alkaline salts to the sparge. In fact don't ever do that, period. If you use Bru'n Water, note that it will tell you the alkalinity of the sparge water after acidification. Instead of calculating the acid addition based on pH, aim for alkalinity between 0-25 ppm, and you'll be good even in a fly sparge. Once the buffers in the mash are set up, they really don't want to pull too far off the mash range.