How to interpret thermometer sensor reading values?
At 25°C the 12-bit extended register should hold 1852 (all numbers are base 10). 

For each degree C above 25°C the reading should decrease by an average of 9.05. So for 45°C the reading should be 1852-9.05*20 = 1671. Similarly it will increase by 9.05 average for each degree C below 25°C

These are nominal numbers and specified as being uncalibrated, so they will vary from unit to unit. 

The nominal (uncalibrated) temperature T in °C for a given count x is: 

T \$ \approx \frac {1852-x}{9.05} + 25 \$