One factor that makes it difficult to gauge door-to-door coverage is the actual "coverage" always has been wildly uneven.
If you live near a kingdom hall and/or are of "nice" disposition, you will likely have lots of JW contacting you which has little to do with the coverage of the territory; it's just that you are in a convenient location or are an easy "target" .
In my old home congregation, a supply store was located around the corner from the kingdom hall. back in the day, the friendly store owner told me (with a mock exasperated tone in his voice) that he had more Watchtowers and Awakes left at his store each week than copies of newspapers he sold daily (back in the day when people bought and read papers). He was a friendly guy and his store provided a handy starting point for counting time. He liked the JWs who often made purchases so it was no problem to him.