I have found that an effective way to measure progress is to keep track of correctly produced and stimulable words. The kids and I have customized a definition of what stimulable means.
It is not correct, but not completely misproduced. If I have a group of students, I will give one student at a time a turn and have the other students in the group work on ear training and listening to correct, incorrect and stimulable words. This helps engage all of the members of the group. I even assign a student secretary to write in the correctly produced words. These words are used for warm-ups, and sent home for homework. I date the list and continue to add to it each session. It is a great visual motivator to watch the list of correctly produced words grow!