Thermistor Calibration
Thermistors are nonlinear.  You need a look-up table to account for the non-linearity.  Followed by linear interpolation should the measured resistance fall between look-up table entries. This web page does a good job covering these steps:
http://stratifylabs.co/embedded%20design%20tips/2013/10/03/Tips-ADC-Thermistor-Circuit-and-Lookup-Table/

You should also be aware of noise in your ADC and design.  And how this effects accuracy.  If your application allows for slow responses to change, consider using exponential smoothing: