You will have a lot more information with a probe thermometer. With dry softwood you may be able to start reducing the air with a 300F flue temp, as long as there is good secondary burn developing and the fire is not put out. With hardwood like oak, hickory or locust it may take letting it get a bit hotter, say 400º. This is going to vary with the wood species, split size, wood dryness and draft strength but with a little experimentation you'll find a sweet spot.
