I use 1 tablespoon per gallon. My water is 7.8 (Detroit City water). But I have spring rainwater now, so all set for the season! I only have PH strips to test, no meter, so it is not exact.

You need to test the pH of the water to determine how much vinegar you need. Even two water sources with the same initial pH can need different amounts of vinegar to get to the same final pH; it all depends upon their chemistry (what's dissolved in them).

Get some pH test strips, the ones which cover a wide range of pH, and do some experimenting. Maybe a teaspoon of vinegar at a time, mix well and test. Keep going until you get to the level you want.

BTW, 1/4c per gallon seems like a lot. Not impossible but most waters need quite a bit less. My relatively hard well water needs 4t/gallon to get to pH 5.5.

I agree quarter cup seems like too much. If I was you, I would get a carbon filter. People say it doesn't affect pH, but it filters out lots of pollution. It does wonders for plants, trust me!

Best of luck,
Joe