When I mixed my own experimental developers, I did notice that developer fog doesn't come in small doses: either your negs are more or less fog free, or fog jumps in your face. As a result, even small deviations from the correct recipe can cause high fog levels which make your negs barely printable.
Here is what I would do in your situation:
- Do yourself a favor and get a pH meter. These are dirt cheap and can be gotten from many sources. pH strips are nice for getting into the ball park, but a pH meter gives you real numbers down to 0.1 or 0.2 pH, depending on what you spend.
- Until that pH meter arrives, add more and more Borax or Boric Acid to that dev and test with unexposed film clips until you no longer get fog.