Somebody may chime in with the ohms/watts combination but if you simply measure the current flowing in your selenoid when energized at 6 volts you can calculate the internal resistance
with V/I=R
adding another equivalent R value resistor in series with the selenoid should do it when pushing it with 12volts .
Since the selenoid is used for very short period you are likely safe with a 5 -10 watt rated resistor or try to find a similar resistance ballast in automotive parts .
you could even test it with a common 12 v light bulb acting as a ballast resistor.
jmho
btw do a search on ballast resistor… I see you had posts on this very subject subject back to 2018 !! with lots of feedback
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