The runoff pH is considered the active pH or active acidity (which seems fine btw and is going to be close to what the roots are actively exposed to - you don't really need to do any sort of calculation). Otherwise, to do any sort of accurate calculation for actually changing the pH of the soil, you'd really need to have a buffer test done on a soil sample. You can get that done through agricultural extensions, usually for pretty cheap; the results will come back with a recommendation for a specific product (like iron sulfate to lower pH or lime to raise pH) and the amount you would want to use to achieve a proper pH. Or you could just use trial and error if you are careful enough.
