I have seen those graphs too, Geoff. But they did not consider airflow around the mirror. I think you need a fair bit more airflow to break the boundary layer. The fan also keeps the dew off my mirror when I bring the scope inside from the cold into the warmer house. It used to dew up all the time before I fitted the fan (90mm 4 watt on the 8" Dob).
You cannot have too much power, only too little.
Andrews Super Ultra Deluxe Dob! Now with free wind tunnel!!