Most of my beers where I live now have been astringent (5 or so). Some more some less. I found one of the culprits after lots of research: I have incredibly high alkalinity (400ppm CaCO3). So I have decided to mix my own water with destilled water and the usual salts. Now I am worried about oversparging. Presuming I construct ideal water and calculate my mash and sparge water using the usual method, do I still need to worry about oversparging? If so how do I go about avoiding this on the grainfather? I've never read a detailed technique anyway. Keep sampling the sparge runoff every 5 mins and test the pH using a meter?
Sparging efficiency is, generally speaking, not a factor of water chemistry. By changing your high alkalinity tapwater to distilled water with added salts you should see no difference in sparging efficiency, and the same sparging volume as you used before should work fine without introducing a risk of oversparging.
Using a pH meter on your runoff is also not very useful; it won't tell you anything about when to stop sparging. The tool of choice to determine that is a hydrometer or refractometer. (The latter is easier to use because cooling down a drop of green wort to the instrument's calibration temperature is much quicker than cooling a tube full of wort.) Test your wort gravity regularly during the sparging process; for an average strength beer you don't want to go below 1.008 in order to prevent oversparging.