I find the best way to determine his ranking would be to compare his position with another player who has ran the course in a similar situation, and see what ranking the player had at the beginning of the Australian Open swing. The player I will be comparing with Andy Murray (later half of 2017) is Roger Federer (second half of 2016).
The similarities between these two players which make them ideal to compare are as follows:
Both Federer and Murray withdrew from their half of the year in the same spot in the ATP rankings: Federer- on July 26th, 2016 was ranked "3" in the world with a point total of 5945, Murray on September 6th, 2017 was ranked "3" in the world with a point total of 6790.
Both players will have lost similar point totals going into each's AO swing-
Roger Federer did not defend point totals in: Cincinnati (1000), U.S Open (1200), Shanghai (10), Basel (500), Paris (90), ATP World Tour Finals (1090) = -3890 points going into 2017 Australian Open swing.
Andy Murray will most likely not defend point totals in: Beijing (500), Shanghai (1000), Vienna (500), Paris (1000), ATP World Tour Finals (1500)= -4500 points going into 2018 Australian Open swing.
This would put each's point total going into the swings at - Roger Federer 2130, and Andy Murray at 2290.
Because of these two criteria, we can make an informed judgement that if we were to estimate Andy Murray's ranking going into the 2018 Australian Open swing, he will be somewhere between 16-20 in the Emirates ATP Rankings. The +4 margin in rankings is largely based on the 160 point total difference between Murray and Federer for each's respective year. Roger Federer started the 2017 AO swing ranked 16.