I'm not sure that I've encountered clear population estimates for the period during the initial Christological schisms - especially because sometimes local churches flipped back and forth. For instance Alexandria, Antioch, and Constantinople sometimes switched between Nestorian, Miaphysite, or Chalcedonian patriarchs, and at times the Roman emperors even adopted new and unorthodox positions (such as Monothiletism in the 600s) that were ill-received by all.
In general the churches in Europe and North Africa (at least before widespread conversion to Islam in the 700s) were associated with Chalcedonian Christianity, while the churches in Egypt, the Nile, and Asia were more associated with either Miaphysite or Nestorian Christianity.
For the latter two, the dividing line was roughly Mesopotamia. From Mesopotamia and east into Persia, India, Central Asia, etc. the churches were Nestorian, while west of Mesopotamia the churches were associated with Miaphysite Christology. The main exception were the Byzantine (Melkite) churches of Southwest Asia, but these were usually much less demographically significant.
With that geographic framing, I did find this estimate from the year 1000:
Assuming that most of the Christians in Asia and Africa at that time belonged to non-Chalcedonian churches, this suggests that by the year 1000 a little less than half were non-Chalcedonian.
Source:
Jenkins, Phillip The Lost History of Christianity (p. 69 - 70).