If it was not useful for anything when it was whole, how much less can it be made into something useful when the fire has burned it and it is charred?
I can't make sense of this sentence. I think the intended meaning is: It would be much less useful when it is burned. But if that is the case "how much less useful" shouldn't come separately.
You're right about the meaning: it must be less useful now that it's been burned than it was before it was burned, probably because it cannot even be used as fuel anymore.
Bible language --even modern translations like this one-- is often unnatural, and this is the case here. The sentence can only be understood by inferring the meaning from the context and piecing together that it's a rhetorical question, and the answer you're supposed to infer is that it can only be less useful now that it's been burned.
