In measuring the resistance of the thermistor, it should vary with a little temp change. Does yours vary? how much?
If you have not already, I like to put a small capacitor between the analog input and ground (about .1 or .01mf). It sometime solves strange readings.
I didn't see which arduino board you are using, just assuming one that is 5volt. If it is a 3.3 volt board, then that may require some adjustments.
I am not a math whiz, but I am having a hard time figuring out how a calculation alone can give much accuracy. There are hardware variables that need to be taken into consideration. The 5Volt that is only 4.7 v. The value and characteristics of the thermistor. If the arduino is a 5v or 3.3v processor... I normally calibrate with a good thermometer, using the map() function, and that seems to work well for any hardware variations that may show up.