Its easy to calculate the pH of an acetic acid solution by using the following equation.
[H+] [Ac-] = Ka = 1.8 x 10-5
For a dilute acid and not a buffer [H+] = [Ac-]
The pH is the log10[H+].
An indicator cannot be used to determine a pH if the solution is outside of its range. So if bromcresol purple yields a yellow color we only know that the pH is equal or less than 5.2.
If the pH of a stop bath is 2.8 and we double its volume with water the pH increases by the log102 = 0.3. So the resultant pH is 3.1. The capacity of such a bath is half what the original bath had.