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For 2 watts at 12v you need 0.166 amps of current, therefore 72 ohms of resistance.
Do you know anyone with a multimeter to measure this for you?
I like Rajahs idea of a tapped point. That way all of the battery current is going toward heating your finder and not wasting it heating the pot instead.
Perhaps use a longer than 300mm length for low power and tap at 300mm for high power? If 300mm gives you 2 watts, then 424mm will give you 1 watt ( sqrt 2 x 300)
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