I just found this on google -- an old post on poolforum. It may be the easiest thing to do to get your pH correct. Sorry, but I don't know who posted it since the cached version isn't very easy to read. Note that you have to adjust for the size of the pool.

Here's what I started doing. Its pretty accurate.

I take a one gallon sample of pool water. Then I add one drop of 20% muriatic acid, mix and run pH. I repeat until the pH is what I want. For every one drop of acid I added to get the pH right, I add a quart of 20% muriatic acid. This is for a 20,000 gallon pool.

Here's the calc:

Fact: 20 drops = 1 milliter
Fact: 3785 milliters = 1 gallon

(#drops added X 20,000 gal)/20 drops per milliter)/3785 milliters per gallon = Gallons of acid to add to pool.

Example: (1 drop of acid X 20,000 gal)/20/3785 = .26 gallons acid