According to bleachcalc, 1/2 gallon of acid will lower your PH to about 7.0 but that is an estimate. You are not that far off.

But Kurt is right, with most plaster jobs, you want to keep the PH between 6.8-7.0 for the first week. It saves time in brushing and keeps the plaster smooth.

A true acid start though will require the alk go to 0 and PH drop considerably. Although effective, this is rarely done since it is very risky and can easily damage the plaster if not done correctly.