Technically, you can take spot meter readings and then decide how much to expose the flames compared to 18% grey. There is no right or wrong exposure - old movies like "Gone with the Wind" were shot on the equivalent of a 10 ASA film stock and fires exposed (at f/2.8 often for interiors) as a deep red/orange. If you are shooting color negative film, you have more leeway in overexposure, the fire will hold detail better, so I wouldn't underexpose too much.
The other issue is balance, if you end up exposing a large nighttime fire at, let's say, f/5.6 at 200 ASA, then how underexposed will the people be around the fire? When "Backdraft" was made, they lit sets to a high light level so that the fire could be exposed for a deeper orange color and yet still see the actors. Or you could just silhouette people against the flames.
The other issue is balance, if you end up exposing a large nighttime fire at, let's say, f/5.6 at 200 ASA, then how underexposed will the people be around the fire? When "Backdraft" was made, they lit sets to a high light level so that the fire could be exposed for a deeper orange color and yet still see the actors. Or you could just silhouette people against the flames.
