The most current the fan will draw @ 12 volts is .215 amp. That is less than 3 watts. The purpose of the potentiometer is to add a variable amount of resistance to create a variable decrease of voltage to the fan. The voltage is what control the fan speed. As the voltage drop, so does the amperage drop. You do realize that too much resistance can cause too much of a voltage drop and stall the fan. Ideally it would be nice to have a potentiometer that is linear and have just the right amount resistance to utilize the full sweep of the potentiometer knob without stalling the fan, but I understand that isn't an option for you now. That's the reason I suggested adding a 3 watts resistor (available from Radio Shack for $1.89) in parallel across the potentiometer. To calculate resistance in parallel use the formula: (R1 x R2) divided by (R1 + R2). Because the resistor has a fix value, and the potentiometer has variable value through its span, the resulting resistance will become non-linear. As I have no experience doing this project, and thus don't know the voltage drop you are trying to achieve, the 100 ohms resistor I had suggested may not be the ideal resistor in ohms. I do hope you realize that what we are doing is "improvising". The decision rest with you as to what is necessary.
Right now the 1k is fine, I just wanted to slow down the interior fan to about 9-10v's, while keeping the exhaust @ max. The sweep range of the 1k pot actually is right in the zone. Allows me to turn it down slightly and I almost get a full sweep before it cut's out, so no worries, here, I actually think this fits in exactly where I want it.
However, I will certainly brush up on what you have put down, great info!!!!!!!