That is about the amount recommended by reputable sources. Know-it-all blogs and such recommendations are all over the board.
I acidify with white vinegar. Our city's water pH, as I would think most are, is variable: something like from 7.8-8.6. I err below the low side. That way it can only do good, although maybe not as good as it might. One tablespoon per five gallons.
And just so everyone knows, white vinegar sounds like it would be very benign, but don't let the household naming trip you up. It still needs to be treated with care!
I am keeping an eye out for a good, durable pH meter too. Let me know if you get one, and how you like it, and I'll do the same.
