The reason you have to experiment with values is b/c you want the circuitry that the termistors trips to be fooled by you making the resistance out of spec .. ie .-incorrect at that temp ..this trip point in the circuit is the factor that determines what value change you are looking for in the thermistor ..just as the thermistor has a +/- tol., so does the other end of the driver circuit that the thermistor is activating...
That is why I mention 100k as a base line try..you may even try a 100K variable and when you tweak it to your likeing, measure the VR tweaked value and sub a metal film in its place for final design/hardwired...