My brewing liquor workbook, MpH Water Calculator, is now updated to version 1.2. (Note: the name has been expanded to make the purpose of the workbook more obvious.) The new addition is a worksheet to calculate acid additions necessary to reduce carbonate alkalinity and bring the pH of the sparge water close to that of the mash. Calculations for both lactic and phosphoric acid are provided.
Sparge water alkalinity reduction is desirable in order to help keep the mash pH from rising during the end of a fly sparge. Such a rise is widely believed to be detrimental due to grain tannins being released into the liquor when the pH rises. (This happens because the malt acidity becomes reduced towards the end of the sparge, allowing alkalinity in the sparge liquor to raise the pH in the grain bed.)
A companion paper, Sparge Water Acidification, describing the equations used in the worksheet, is in progress and will be posted here shortly.
Happy brewing y'all, and cheers!