Okay, I looked at the circuit in the FSM and I think you're onto something. The fuel lamp never does come on at full brightness because there is still a lot of resistance in the thermistor even when it's hot. This means that the bulb's filament is cooler and this means that the bulb's resistance is less than it would be if operated at full current. There are several ways to find the correct resistance in that case but your method of inserting a variable resistor will work to find the required value. But be careful because there are several ways this can go wrong. You can burn up the potentiometer, or worse possibly burn up the thermistor, by turning the resistance too low.
My choice would be to use a voltmeter that has an ammeter setting. I'd put the meter in series with the bulb (while the gas is low) and measure the current through the bulb. I'd then remove the ammeter from the circuit and set the meter up to measure the voltage across the bulb with the meter in parallel (again while the gas is low). Now that you know both the voltage (V) that the lamp is seeing and the current (I) running through it you can calculate the actual filament resistance using Ohm's law. V/I = R. I'd go with a little lower resistance than what you calculate because you don't want the revised circuit not to work on a cold day. This is more work but you'll know what the correct value is and won't risk frying something.
