There is no base line for your question. Reason being that there a multi reasons (temp, elements in water) that can effect the pH outcome. You need to add a teaspoon per gallon, then get a pH reading, then adjust again as needed until you acheive your desired results. Best to buy pH up and down. Home Depot sells a kit for 20 bucks that comes with a pH tester and up and down.
There is a shit load of science that goes into trying to calculate this out, and you have to have alot of known variables to do it right.
Here is an idea. pH of neem oil is 6.5, you could add a table spoon per gallon when watering and this should give you a balanced water. Keep watering with this solution and the pH of your soil should gradualy go down.