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Sparge volume is the pre-boil target less the first runnings already collected, plus a small allowance for water retained during sparging. Sparge water is held at 75-78 C to keep the wort fluid without extracting tannins, and pH should be under 6.0 for the same reason.
Sparge volume
Sparge water = pre-boil volume - first runnings + additional retention
Above that, polyphenols and silicates leach from the husk, giving astringency and haze. High pH in the last runnings has the same effect, which is why final runnings gravity is monitored.
When the runnings fall below about 1.008-1.010 specific gravity, or pH rises above 6.0. Continuing past that point extracts more astringency than sugar.