Next time you're at the drug store or vet, ask for a little 1 mL plastic syringe doser. You really need this level of accuracy. Then figure out how much acid you need to drop your water to the desired pH. For example, I found that I need like 0.6 mL/gal to get down to pH of 6.0 for sparge water. Don't use my numbers since it depends on mineral content of your water.
If you're using lactic acid for your mash (instead of acid malt) then you really need to use one of the spreadsheets and develop a base of experience with it.