The acceptance criteria should be determined in the specification that made you measure the ferrite in the first place.
The only codified criteria I can think of offhand is filler metal testing in Section III.
Codes try to stay away from this because it is so variable due to heat input, AND, becasue of the great variability in services.
You do not want too high ferrite in high temp services where sigma can be a concern. You do not want ANY ferrite in cryo services where ductile to brittle transitions might be a concern. Most services appreciate some ferrite for microfissuring. But there may be some engineering criteria where fully austenitic is preferred.