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So the software program tells you to add such and such to mimic the water, of in this case a London Porter, well don't you still need to add an
acid to the sparge water to bring the PH into an acceptable range
Simple answer: Not really, no. Especially for a beer like a porter (or anything else darker than about 10-15 SRM). I do manage my sparge water for something like a Kolsch or a Hefe. But, you are unlikely to have any issues with something as dark as a porter. For this to be a concern with a porter, your sparge water would need to be 150 ppm CaCO3 or greater.
If your water has that much buffering capacity, then I would just cut it with 50% distilled water.
Detailed answer: I'm a firm believer in the "measure and then adjust" approach. Acid is an adjustment tool that should only be used based on a measurement. If you are dead set on acidifying you sparge water, add a few drops of acid (lactic or phosphoric) to bring the pH below 6.0. Then monitor the pH of your runnings, and adjust as necessary. Both acids can have flavor impact (good and bad), so I do try and minimize their usage. If I can accomplish my pH goals without acid, that is preferable.
Tom is correct; it doesn't take much. Our water here (Charlotte) is very low-mineral content, but they boost the pH with something that raises pH to 8.5 to 9.3.
So I use Palmer's worksheet to adjust the mash RA, and just one drop of phosphoric in four gallons to "bust up" the very high pH of the sparge water. While the pH is artificially high, it does not have much staying power (buffering ability). That drop gets the pH down below 7, and the mineral-adjusted mash can easily keep the pH in a good range from there.
So you may want to verify the starting pH and whether it's naturally high/low, or artificially adjusted like it is here.