I have some pH indicator strips and I used to just add enough indicator stop bath to the water to get the pH down to about pH 5 or 5.5. I think it's somewhere around 10 mls into 1 L. I haven't found the stain of PMK to be afftected by this level of pH.

If one is doing non-stained films and wants a stop, I've been using the following (I haven't tested it with PMK yet as I did with the dilute stop I mantioned above.)

After my big flap with Bill Troop last year on photo.net about stop baths and alkaline fixers, I've been using Bill's stop bath formula from the Film Developing Cookbook. It's a buffered acetic acid solution, made from acetic acid and sodium acetate. It buffers around pH 5 as well, but has a much higher acidity than unbuffered acetic stop baths - even if they have a lower pH. The increased acidity will speed up the stopping action of the stop bath, down to only a few seconds, according to the FDC. And since I use Rapid Fixers, which have a pH of about 5.5, I don't have to worry about all that emulsion swelling one should get by putting the film back into an alkaine fix. (Even though it really wouldn't be that much with a near neutral, alkaline fix.)

Bill talks about this buffered stop being "expensive", but for film, I wouldn't worry about. Certainly much better than spending all that time doing multiple water washes and then not really stopping the development anyway.