The screw on the baffle is to regulate the amount if bypass. If the smoke temperature in the chimney is to low you increase the bypass. If it is high enough you decrease the bypass, possibly until the baffle is completely closed.
The rule of thumb is that the smoke gas temperature one meter down from the top of the chimney should be at least 75 C in order to avoid condensation that could, in the long run, potentially ruin your chimney.
