Actually PH doesn't really correlate to the amount of acid needed to neutralize different bases. Developers tend to have buffering agents in them as well which makes thing even more complicated.
You can tell when the base on sheet film is properly neutralized, the sheet is slippery in the developer (bases always feel this way) and when you put it in the stop, the slipperyness rapidly dissapates. If this doesn't happen, you know that the stop is spent.