@Rudeofus in one of the posts above gives a pH of 5.5, which will be an effective stopping and cleansing bath.
The pH value of 5.5 is obviously not the gospel, but allow me to explain it a bit:
- Going above 5.5 would limit its activity as stop bath. CD-4 shows measurable developing properties at pH 6, but I don't expect it to be much of a developer at this pH. Either way, 5.5 is below 6 so we are on the safe side
- Going below 5.5 would create Sulfur Dioxide smell with the sulfite in it, so I would not go below 5.
- Commercial rapid have pH between 5.2 and 5.5. Their concentrate lasts as long as C-41 concentrates, and few people report Sulfur Dioxide smell from these rapid fixers.
I would conclude, that target pH is somewhere between 5.0 and 5.5. You can probably go lower if you have an insensitive nose, and go higher if you use it strictly single shot.