Attached is a graph of Fan Voltage vs temp for various values of R2. My suggestion would be to replcae R2 with a 1k pot.

The graphs are theoretical, based on the formula for the regulator and the thermistor resistance/temp values that Franks had. Some errors are evident, but the graphs should be good enough to get you started. Some of them show voltages over 14 volts. Obviously this isnt really going to happen - they will level out at Vbattery minus some L200 specific value (2V ?? - I'm guessing here.)